It's probably a plugin issue - see Video or audio does not play.

MySpace and YouTube both use Managing the Flash plugin to embed their sound-containing content, so start there. First download the Managing the Flash plugin uninstaller and the latest installer. Then close Firefox, run the uninstaller, then the installer (see the Managing the Flash plugin article for other Managing the Flash plugin troubleshooting).

You might also try re-intializing the plugins database.

If that's no go, repeat the uninstall/ re-install routine for Using the QuickTime plugin with Firefox.

If that's no go, see if it works with a new profiles.

Hope it works!

P.S. There are separate installers for the Flash ActiveX and Flash Plugin players. Bo gave you the "install_flash_player.exe" link for the Flash installer for Firefox (if you go to http://www.adobe.com/go/getflashplayer in Firefox, it includes the same download link, plus instructions).

Important: If you use the "uninstall_flash_player.exe" Flash uninstaller that Bo posted, it will also uninstall the Flash ActiveX player for Internet Explorer, according to Adobe's instructions, How to uninstall the Adobe Flash Player plug-in and ActiveX control (more on that here). You will also need to reinstall the Flash ActiveX player (you can do that by visiting the getflashplayer page in Internet Explorer).

I am having the same problem sound works with IE but not Firefox.
I'm glad BO got positive feedback from Jamie. This must have been because Jamie's problem was non typical.
Searching the internet for a solution to this problem is a nightmare. Everybody has different idea's on how to fix it!
The normal diagnostic routine which is posted somewhere on the mozilla forums doesn't work either.
Some guy on a forum far far away said something about cut and pasting some registry values.
(I havn't tried this method yet so it's still just a complete guess like BO's.)
It goes something like this:

1. Go to start menu run and enter REGEDIT and press return
2. This will start the regestry editor. Goto the file menu and select export and save the regestry as a file, make sure you have Export Range / All selected at the bottom of the save file window. This will save the whole regestry as a file, remember where you save the file... You have now backed up the regestry. Now close the regestry editor
3. Copy all the following text starting from Windows Regestry Editor Verion 5.00 by highlighting it, right clicking and selecting copy

Windows Registry Editor Version 5.00

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Drivers32
"midimapper"="midimap.dll"
"msacm.imaadpcm"="imaadp32.acm"
"msacm.msadpcm"="msadp32.acm"
"msacm.msg711"="msg711.acm"
"msacm.msgsm610"="msgsm32.acm"
"msacm.trspch"="tssoft32.acm"
"vidc.cvid"="iccvid.dll"
"VIDC.I420"="i420vfw.dll"
"vidc.iv31"="ir32_32.dll"
"vidc.iv32"="ir32_32.dll"
"vidc.iv41"="ir41_32.ax"
"VIDC.IYUV"="iyuv_32.dll"
"vidc.mrle"="msrle32.dll"
"vidc.msvc"="msvidc32.dll"
"VIDC.YVYU"="msyuv.dll"
"wavemapper"="msacm32.drv"
"msacm.msg723"="msg723.acm"
"vidc.M263"="msh263.drv"
"vidc.M261"="msh261.drv"
"msacm.msaudio1"="msaud32.acm"
"msacm.sl_anet"="sl_anet.acm"
"msacm.iac2"="C:\\WINDOWS\\system32\\iac25_32. ax"
"vidc.iv50"="ir50_32.dll"
"wave"="wdmaud.drv"
"midi"="wdmaud.drv"
"mixer"="wdmaud.drv"
"VIDC.WMV3"="wmv9vcm.dll"
"VIDC.VP40"="vp4vfw.dll"
"msacm.voxacm160"="vct3216.acm"
"MSVideo"="vfwwdm32.dll"
"MSVideo8"="VfWWDM32.dll"
"wave1"="wdmaud.drv"
"midi1"="wdmaud.drv"
"mixer1"="wdmaud.drv"
"aux"="wdmaud.drv"
"vidc.VP70"="vp7vfw.dll"
"vidc.X264"="x264vfw.dll"
"VIDC.FPS1"="frapsvid.dll"
"vidc.VP60"="vp6vfw.dll"
"vidc.VP61"="vp6vfw.dll"
"vidc.VP62"="vp6vfw.dll"
"vidc.DIVX"="DivX.dll"
"VIDC.UYVY"="msyuv.dll"
"VIDC.YUY2"="msyuv.dll"
"VIDC.YVU9"="tsbyuv.dll"
"VIDC.DRAW"="DVIDEO.DLL"
"VIDC.YV12"="yv12vfw.dll"
"wave2"="wdmaud.drv"
"midi2"="wdmaud.drv"
"mixer2"="wdmaud.drv"
"aux1"="wdmaud.drv"
"wave3"="wdmaud.drv"
"midi3"="wdmaud.drv"
"mixer3"="wdmaud.drv"
"aux2"="wdmaud.drv"
"VIDC.MSUD"="msulvc05.dll"
"wave4"="wdmaud.drv"
"midi4"="wdmaud.drv"
"mixer4"="wdmaud.drv"
"aux3"="wdmaud.drv"

4. Goto start menu / run again and this time type in NOTEPAD and click return
5. Now right click on the notepad window and select paste. Then select save from the file menu and save the file, make sure the filename you enter ends in .REG Remember where you saved the file and close notepad
6. Finally, navigate to your saved file and double click on it. You will see a dialoge asking whether you are sure you want to add the information from the file to the regestry, simply click OK. Once this is done, then try the website you have been having trouble with and see if it works. Any problems, simply double click the backup file you created in step 1 and this will restore your regestry back to what it was.

I just wonder if it's a coincidence that SUDDENLY so many people started to experience firefox not having sound while explorer does. Never happened to me until this last couple days. I can't figure out the fix, so I guess I'm gonna be using explorer from now on.

I've been having the same problem - no firefox audio but running IE (or in an IE tab) works fine.

I fixed mine by installing the Firefox 3 beta into a separate directory. I tried playing a vid on youtube - worked fine, so I shut down and restarted my old firefox. Whatever was causing the problem originally seems to get taken care of by the new version...

Not sure if it'll work for everyone but it's a lot quicker/easier than messing with the registry and a good excuse to play with the new version of Firefox besides. :)

I ran into this problem just recently and fixed it surprisingly quickly. You might try this before you go uninstalling/installing. In my case something had changed my audio settings, so do a quick check to make sure they are still set correctly.
Go to Control Panel, Sounds, Adjust the system volume. If the Device Volume in the volume tab is set to low:
a) Select Audio Tab, set default device to your sound card (something set this to blank on my system!)
b)Press Volume button, uncheck all checkboxes
c)Select Sound Tab, set sound scheme to windows default

click ok and exit the control panel. Test to see if you have sound in Firefox.

HTH

The mixer thing is what fixed it for me.

I installed 3.1 and am tired of trying to fix this problem. I am NOT a computer geek, and in ill health, and have been trying to fix this for nearly a week, now.

I am filing a formal complaint with the New York Attorney General, as firefox seems to be totally ignoring the fact that they have a bug, and are offering no fixes for it.

I am SICK of all the different solutions! NOTHING works, okay? I can't get into any complicated fixes, cos I am just too ill to concentrate on long and complicated (a whole page of "do this and do that and do a million different other things).

I'll let the state of New York try to help me. Last person told me very snottily to take my issue up with Realplayer! (Which works a 100% by the way).

Firefox USED to be good, and for some, I'm sure it still is, but there's thousands of us having this issue and Firefox is getting worse than Microsoft.

They have DESTROYED my ability to hear sound on the internet.

I have tried older versions from 2.0 to 3.0, I've tried switching to Opera, IE, etc. I've tried updating Adobe flash and fiddling with ActiveX, I've updated windows,

Quite frankly, I'm done uselessly jumping through hoops.

I'll let them tell New York state why they refuse to address New Yorker's issues, and why they refuse, in the face of so many problems, to acknowledge that they have a bug in their system.

Incidentally, in June, before I switched from Mozilla to another browswer, ALL my audio--including Realplayer, worked 100% on Mozilla...but suddenly, I update Mozilla, and it no longer recognizes Realplayer?

I say, Mozilla's got some issue with Realplayer, that they don't want people to know about...why would it work on the old version, and not the new one? That does NOT make sense.

I'm having the same problem and it's really starting to piss me off.

I downloaded Firefox 3 and all of a sudden my browser is muted.

I've Tried just about EVERYTHING that people have come up with and nothing seems to work.

There's got to be a quick fix to something this simple.

Thanks "jipsi" your solution fixed my problem with poor audio!