Youtube without sound in Firefox 25

I am not getting any sound in youtube under mozila firefox. However the sound is coming perfect in IE. The latest version of flash player is installed in both the cases. What should I do?
My Rig

INTEL p4 HT 2.8GHz with 3 GB RAM and Intel 865 GBF MB(original) and win 7 Home premium

What about non-youtube sound??

check in audio mixer if volume level is full for firefox.

W

YouTube Drops Support for 1080p Streams in the Regular HTML5 Player

For now, only Chrome, Opera and IE11 for Windows 8 support Media Source Extensions. If you're using Firefox, Safari or an old version of IE, you're out of luck.

The same version of firefox is running youtube fine on my laptop. IE version is also 8, in the desktop which is running youtube with sound.

W

*support.mozilla.org/en-US/questions/966369

Finally, after hundreds of hours I fixed it and Firefox plays sound in HTML5 in everywhere, YouTube (HTML5 mode), SoundCloud, Bandcamp and etc.
You have to: Right-click on your sound/speakers icon in the system tray and select Playback Devices, go to Speakers properties (the one which is green-ticked, also disable the red ones, if any), then go to Enhancements and tick Disable all sound effects. And that's it, restart the browser and enjoy.
In short: Volume mixer -> Playback devices -> Playback tab -> Speakers properties -> Enhancements -> Disable all sound effects -> Restart browser.
I use latest stable Firefox 23.0.1 on Windows 8 x64 (but Firefox is 32-bit).
Also, if didn't help, make sure you have media.windows-media-foundation.enabled set to 'true' in about:config.
You should also have some decent codecs installed in your machine, I have VLC Player and it comes with lots of in-built codecs. Also make sure you have VLC Web Plugin enabled in Add-ons -> Extensions (Always activate)

So the unanswered is who's problem is that? Is it Firefox? Or Windows 8? Or my Realtek audio drivers fault? (Anyway without disabling sound effects, HTML5 works fine in any other browser, so I blame Firefox for this nonsense for extra configurations in Windows audio settings)
So, pretty much it.

Cheers, rhazor

Hmm, will give it a try when I will be at my native place again. At present doing fine with IE. Perhaps this is the reason I do not want to upgrade any software/hardware as long as it is doing fine with my purpose.