Here's something interesting, these SID tunes were recorded using two different SID chips and mixed together to get a nice echo effect:
That's not entirely true; while they were recorded and mixed together (one SID model in the left channel and another in the right) the echo effect is done programmatically entirely on the C64. The third voice oscillator is read by in a fast interrupt and fed back a few hundred samples later into the master volume register of the SID (which is a a common trick to play 4-bit samples on the C64).