Themes and Tone
Attach music to key characters or themes in your game to create musical themes. Play a specific tune whenever the bad guys show up, and you can allude to them later just by playing the track. Play a track whenever the characters are in danger of defeat, and the players may learn to dread the sound. Play a track when magical rituals are performed, or when bizarre aliens are discovered, and you might have a theme for wonder or discovery.
You can create a distinctive character for your adventure or campaign by mashing up musical styles with different visual descriptions and games. A D&D campaign with a soundtrack made up of Nordic death metal feels different from a campaign set to percussive cues from the new Battlestar Galactica; those two campaigns are likely to be about different things, if the music that inspires them is any indication. Consider a campaign that mixes traditional medieval music, used for moments of rest and courtly intrigue, with driving rock music for glorious over-the-top battle scenes. That campaign certainly tells players more about what combat and intrigue are meant to feel like in the game world than a campaign without music does.
Be careful, though, that you don't accidentally create dissonance between your imaginary mind's-eye visuals and your auditory cues. Music often moves forward in time more poignantly than it moves backwards. Bear McCreary's great scores for the new Battlestar Galactica use traditional instruments and percussion to create an epic, timeless quality to a show about spaceships and robots. On the other hand, electronic synth tracks laid over medieval adventure can be jarringly anachronistic.
Music sends signals to the players. Your favorite action cue sends the signal that it's time to engage the combat mechanics. An enemy's theme suggests that the foe is involved with the current scene even if he isn't physically present - menacing if the characters are battling disguised minions, intriguing if the characters are investigating a crime scene.
If you're trying to send a signal that reminds the players that "Violence in this world is bloody and epic and emotional" or a signal that says "Action scenes in this game are rocking, high-flying spectacles," make sure your music is sending the same signal.
Mismatched music can create a moving and provocative atmosphere ... or it can be accidentally hilarious. Sometimes a music choice goes unnoticed and, worse, it draws the wrong response from your audience. So it goes. Move on to the next track in the playlist. Keep the game moving.