AABACA—good deals on music software
midi-classics.com—good deals on music software
Sibelius official website (but the program is cheaper at the sites above)
Logic Express (the “lite” version)
GarageBand (Logic “lite-lite” but still powerful)
Band-In-A-Box official website
MacGamut cross-platform despite the name
Amazing Slow Downer cross platform
Dreamweaver official website
a brief discussion of MIDI and Digital Audio by Dan Cragan
a more detailed Introduction to Digital Audio by Christopher Dobrian
The Logic user manuals also have a quickie non-intimidating rundown of audio and midi basics in the index.