Another sound experiment was to use the pic as a pass through. Put music in on one side and get it out at the other side. The music is fed into an A/D converter and converted to a digital signal. On the other side of the pic it has to be converted back from digital to analog.
http://members.home.nl/b.vandam/lonely/pagina44.html
http://lillith.sk.tsukuba.ac.jp/~kashima/games/bmconv/index-e.html
midi signals to computer serial port interface
http://www.ee.washington.edu/conselec/A95/projects/esimpson/midi.htm
using PIC Microcontrollers and Basic Stamps
http://www.audiomulch.com/midipic/
http://www.imagesco.com/articles/hm2007/SpeechRecognitionTutorial02.html
high performance transformer balanced input stage for Crystal A/D converters
http://www.jensen-transformers.com/as/as043.pdf
for Crystal 18 or 20 Bit A/D Converters
http://www.jensen-transformers.com/as/as062.pdf