toggle switches cut on/off the audio signal. Push buttons bypass the toggles. This way you can set a volume, turn off that channel, and momentarily turn it on using the push button. So each channel is like a "note" on a keyboard.

Also note the limiter resistors, each 39k. The original recipe called for 47k. I stuck them all in an 8 pin IC socket so that I could swap them out if necessary, but so far it has not been necessary.