Mostly, pre-recorded music from video games, films etc.

Breaking that down into various categories of music:

LOCATION/CULTURE - e.g. the Rivendell music in LOTR
EVENT - e.g. the hero uses powers for the first time; an escape scene; a boss fight
MOOD - e.g. joyous victory, tragic defeat, love, fear

Things harder to do at home:

CHARACTER - Often adapted for context e.g. in film, Captain America picking up Thor's hammer.
IDEA - a theme for the core question/subject of the campaign e.g. a theme for order and a theme for chaos; a theme for moral compromise...

Some questions about music:

- Should music be from the time period (e.g. medieval stuff for fantasy) or anachronistic (e.g. heavy rock) or filmic (e.g. big orchestral sweeps). Worth noting that a lot of 'genuine' medieval music comes from an absurdly broad sweep of time, much like many things claimed to be 'true' medievalism.
- Pre-recorded music often carries certain issues e.g. Orientalism, Eurocentrism. How do we avoid it?
- Resources an issue for people - easy enough to play pre-existing tracks, but takes more to make music for a game. Electronically synthesise tracks?
- Pre-recorded music usually allows people to have more developed music - but equally means it's less responsive to what's happening in-game.
- Live music can be more reactive, but also smaller (and, obviously, not for a home game). In a live show, should it be participatory?

Music can be used for worldbuilding, if thought put into it.