…like, reading an .mp3 file of, say, The Beatles’ “A Hard Day’s Night” and using it in some way as data in the game?
Load it through a DataInputStream and read level data off that.
It won’t be pretty, but that’s what you’re asking for.
If you want something else, be more specific.
You can decompress to raw PCM float data (from WAV/OGG/MP3), i.e. in the range 0.0 … 1.0, and then potentially do something interesting with the data.
A more practical application is to use a Fast Fourier Transform to split the frequencies of the sound data, i.e. to create a visualizer that reacts to bass hits. Or you can use onset detection:
https://code.google.com/p/audio-analysis/
If you just want to play the audio files in your game, you should use a solution like TinySound:
Or if you are using LibGDX, it should be fairly simple to include audio in your game.
Actually, I was thinking more in terms of a game such as Audiosurf, which lets the player select a song from their music library and then builds the level that the player will play based on that song.
that would be the “potentially do something interesting with the data” part
You can use Paul Lamb Sound Library, it’s very easy to use, it has tons of nice backends for both JavaSound and OpenAL, it supports several audio formats including Midi, Wav and Ogg. JOAL has a build-in wav decoder too.
Or require MP3 and then you get data in the frequency domain already. Requires a bit more effort, perhaps, because decoders tend to be all-or-nothing.