Either the default sound track or none. In a lot of games I get tired of the music so I turn it off. It either gets annoying or I turn it off to make the game feel more realistic and enjoy the environment sounds.

In some MP games sound plays an important role and you can use it to your advantage. Like hearing the enemies around the corner, etc. So I'll make sure to have the music off then and the sound effects cranked.

The only video game that I've played music in the background was WoW. It's a grind feast so I think 99% of the WoW population does.