I recently started having issues with sound on flash multimedia files on the internet while using one of my pc systems. I had just recently updated my Adobe Flash Player 9 files and assumed that was part of, if not all of the problem. I was also able to pinpoint MY problem to Firefox and not Internet Explorer 7.
I turned to Google and found that this problem is relatively widespread and found some possible solutions for the problem. I was able to fix my particular situation in the matter of a few minutes. There is a thread on Adobe.com in their forum that offers four possible solutions from user beezlebob666. The first involves replacing the Wavemapper driver that may have been lost or damaged in the installation or upgrade of the flash files. I checked to make sure mine was there and it was, had it not been there (and this was the problem), this particular fix takes only seconds. I would recommend backing up your registry before making any changes to it.
The second solution actually fixed my problem. Somehow I had damaged flash content on my system and this solution suggested downloading the CCleaner program (it is free) and removing cached flash content. I already had the program and all I did was set it (by checking boxes) to only scan for flash files and then deleting them once they were found. I restarted firefox and now I can hear my YouTube Viral Videos again.
The third fix is to replace your possibly missing or outdated codec. The easiest way to describe a codec is like this: your audio and video files are shrunk down to be sent quickly across the internet and you need the files to be un-shrunk to view or listen to them. To do this, you have to have a matching compressor and de-compressor. This fix is getting you the tool you need to de-compress these files. They suggest downloading and installing the K-Lite codec pack that too is free and simple to install.
The final fix is to essentially give up and revert back to the then Macromedia Flash Player 7 until you are able to figure out a way to make the latest flash player work for you. Some sites will not work for you as they require the use of the latest flash player, but then again some will and you may be willing to gain some working sites rather than none. Be sure to turn off automatic updating as this player will continually try to get the latest player.