I've heard good things about PortAudio:
http://www.portaudio.com/

I sorta-know a couple of guys who use libao, don't know much about it:
http://xiph.org/ao/

Maybe libsndfile for file reading/writing?
http://www.mega-nerd.com/libsndfile/

Lurking on the music-dsp list might also be interesting. The discussion there is usually way too advanced for ...everyone sane... but it can be fun. There's also code snippets at:

http://www.musicdsp.org/

I'd recommend flipping through various DSP books, and reading at least the chapters dealing with the basics of sampling, sample rate conversion, etc. Check the local libraries.