Hi tim.vocam, ' Thank you for your question, as far as I understand mp4 files uses its own codec independent of Firefox's built in player. However if you drag and drop an mp4 file into Firefox it should play. Does this also happen on other browsers if its an embedded webpage?

These are also good references for further troubleshooting the audio for win 7 and 8 [Fix common audio and video issues]] ?

Some issues that are being worked on for an mp4 player: [Bug 908503 - Include an MP4 demuxer in Gecko for video] [Bug 984223 - Support H.264 hardware codec decoder in WebRTC]

Hi guigs2

Yes, we tried to drag and drop an mp4 into Firefox, the audio still stuttered.

It only happens on Firefox on WIndows 7 and 8, Windows XP and Mac are working fine.

It works fine on IE and Chrome too.

Hi

For some reason Firefox on WIndows 7 & 8 does not regonise lower audio sampleeate.

We tested 2 mp4s, one coded at 44.1khz, the other coded at 22khz. Firefox doesn't pick up the 22khz.

I am not sure if it somethin to do on Windows 7 and 8?

No audio at all with the lower khz huh? If it was the video player, this is the chart for OS support: http://en.wikipedia.org/wiki/Comparison_of_video_player_software however the low 22.05 kHz is a tested half bitrate.

By default the built in HTML5 player is used, and for windows h.248 is supported. 

For a test, turn off your video accelerator add on, update your flash and try to play the video with the lower and higher codec.