If your sound card drivers have multiple modes, such as Creative's Sound Blaster and many others, you may have speaker settings turned down in game mode thus whenever your PC plays the game, your audio fades. If this is the case, then you'll have to cycle through all of the modes and turn all of the speakers to the same levels.
Also, while you're looking through the settings, try turning off any special effects settings you come across. I had a buddy who had some real bad echo whenever he played music over any media player program, but never with anything else like running a game or watching videos or listening to streamed music. Turns out his sound card had some fun little enhancement section and things were selected at some point which created the echo effect. We unchecked all of them and his problem went away.
Another trick you might want to try out is going to your speaker properties from your control panel. There should be an advanced tab in the speaker properties box where you can access a couple features, one of which is "Allow applications to take exclusive control of this device." Another is "Give exclusive mode applications priority." Uncheck one or both of those boxes and try to play again and check youtube while the game is running. This may fix your problem as well, but try the other way first.
Edit: Also, the latest drivers aren't always the best, for example, my laptop, when I upgrade the conexant audio HD drivers to the (recommended by Conexant and Windows update) conexant audio HD 2 drivers... I lose all sound until I search for new hardware, this behavior is also including when I've just started the laptop up each and every time after performing the update. So updates can be bad.
