I'm a great proponent of using music in my games and have been collecting all sorts of tracks from PC/Console games, movie soundtracks and sound effects for several years. It's nice to have music and sound effects in your game because it becomes an added 'voice' in describing where the players are or what are they doing.
However it becomes quite a hassle to try to organise and play all the sound effects and music together without it sounding too repetetive. Sound effects files tend to be short and when played on the loop sounds monotonous and boring. It gets even harder trying to mix it with music because all you hear by then is the sound effect turning into a beat for the music.
Then came along Syrinscape, a cool tool that I found over at ENWorld.
It is a simple sound effect and music mixer/player where you can organise the timing and volume of the sound effects that you want. It's free to download (but requires a free account) and allows a certain degree of customability to add in your own music and effects as well.
This is very impressive as a DM/GM's tool because once you've arranged the timing of sound effects to how it should be played with the music, you could just leave it on its own as it loops (which you can also decide to turn off). Stopping the music and effects is also just a click of a button away instead of having to close all the players playing each track simultaneously.
Here's a short video of how it works and you could see how easy and useful it is!
(taken from the Syrinscape website)
Speaking of sound effects. The Gnome Stew is organizing their first contest where the prize is 5 studio-quality sound effects of your choice! You should really check it out here!
As for music, Stargazer's World has just gave a plug to a German group that compose tracks for roleplaying; Erdenstern. You can download some of their free tracks at their website. Don't worry, they don't sound like Rammstein.