Now working without the crystal. This is using a Teensy and one of the pins to generate a 4mhz signal. I will post a video in coming days, hopefully, as soon as I have some CoCo BASIC code talking to it.
The green and black wires running off of it wold go to a headphone jack that the cassette cable could plug in to so you could feed Sound back to the CoCo without needing to use an external speaker. AUDIO ON!