Multiplayer games, particularly shooters, silence is best so you can hear gunfire and footsteps, particularly for Counter Strike, hearing the click of an awper going into zoom can mean life or death in some circumstances, also hearing people around the corner before they get to it can yield an easy kill from a noob who doesn't know how to hold Shift, or ones who fail to check their corners.
Also WoW, its just distracting, hearing teammates is crucial.. if I'm grinding, then usually I'll watch TV or listen to better music (not that the WoW soundtrack is bad by any means, I just have better music).
Ohh, and 8-bit chiptunes, really, really not a fan, some are catchy, but I don't need to hear it throughout the entire game on loop thanks (not all 8-bit games are guilty of this, but there are a lot of them).