The reason why magically sound started to play when you disabled one of your sound cards is that ALSA nolonger could use the other card as the default.
So if you run into this issue make sure you verify which card became the default. I had the same problem in fc11 with flash player 10. Even though every where in KDE4 I had specified to use my SB Live card in the browser I did not get any sound with youtube or any flash based site for that matter. To fix this go to:
/etc/alsa and in the conf file set your desired card as your default.
In my case there was a virtual Pulse audio card. So what I did was to set the default again to my hw card (see below):
Also make sure your mixer settings aren't muted but I guess if you were looking to solve this you probably already did all the obvious things
After setting your default properly the flash player in firefox should play again (or for the first time).