I'm trying to build a project that takes an audio input. I took the advice from this how-to and bought a cheap USB sound card. I'm pretty sure mine has only mono input, which is fine because the input is going to be from a record player, but you might need to look to a robust model or a dedicated microphone input device if you want higher quality sound.
The next thing I need to figure out is how to capture the audio source and decode it to a file that can be played back. That I haven't yet figured out.