I am having a problem where the audio levels fluctuate when I alt-tab to and from games.

I like to play music using youtube or pandora.
Usually everything sounds fine, until I launch a game, in which case the music levels drop and seem to lose quality slightly.
They then continue to fluctuate randomly while the game is open, usually when I join or change servers.

It happens most notably with CS:GO and League of Legends.

I use a Logitech G35 USB headset, but have not had this problem on other machines.