The Itti Bitti uses the PIC's built-in A/D converter to sample audio signals, and then it uses a really crude D/A converter made with resistors to send the sound out. Current it just passes the audio through itself, but it could do more! The sound is sampled and played back in 8 bits.
http://www.andrewkilpatrick.org/mind/itti_bitti/
A transformer is an electrical device that allows an AC input signal (like audio) to produce a related AC output signal without the input and output being physically connected together.
http://www.shure.com/ProAudio/Products/us_pro_ea_audiotransformer
JT-MB-C in Low Noise, High Isolation Line Receiver for Low-Z Sources
http://www.jensen-transformers.com/as/as035.pdf
This project describes the construction of test equipment for double-blind or ABX testing of source components - preamplifiers, tuners, DACs etc. or even, if that is your particular vice, interconnects.
http://sound.westhost.com/abx-tester.htm
The ESP Sound Impairment Monitor is a method of determining just how much your amplifier modifies the original signal. The first version is designed to be built into an amplifier circuit, and although quite simple in concept will indicate if any modification is made to the signal by the amp, for any reason at all.
http://sound.westhost.com/project57.htm