I already had a Mackie 0402VLZ4 mixer and a Beaglebone Black, so the only thing I needed was a way to get audio in and out of the BBB. The cheapest option was the Behringer UCA0202 which can be had for about $30.
The audio from the microphone goes into channel 1 of the mixer which is panned hard left, then from the left output of the mixer into the audio interface. The audio interface output then goes back into the right tape return channel of the mixer. I could go from the output of the audio interface directly to the PA but since I’ve already got the mixer I might as well use it to provide a balanced TRS connection to the PA.
Here’s a diagram:
