Audio is not all about music, SFX are also quite important and can greatly enhance a game. You can find SFX sound banks with unrestrictive licenses online, or generate them with a tool like sfxr.
As for music, it's fun to learn how to make your own, but if that's not your cup of tea, you can find music for your games in places like opengameart; just be sure to read the licenses and credit the authors appropriately. Choosing a good background music can make the graphics and the audio come together to make a great atmosphere, this game for example could probably use a catchy chiptune soundtrack, with crunchy sfx.