Well due recent changes I noticed that using flash on firefox was hijacking the sound on my system.
If you aren’t a careless internet surfer and still want to use the latest amd64 flash version that didn’t have the sound problems you can try this.
then download this flash release.
Extract libflashplayer.so into the directory we just created and restart firefox.
Some other people say they have benefit by setting an environmental variable but that didn’t work for me.
I’m not sure if the issue is only related to x86_64 yet.