Each genre has its own soundtrack catalog of nearly 200 tracks. The current track name shows in the top bar with a speaker icon — click the mute button to toggle audio.

Click the track name in the top bar to open the music player dropdown. From here you can:
Music changes automatically based on context. When you enter a building with assigned music, that track plays. When you walk into a different biome, the ambient track updates. When you leave a building, the map music resumes.
You can pin specific tracks to buildings or biomes: