I'm testing a small interface for some musical and technological projects which has an instrument input jack on one side and no audio output, just USB on the other side. This IF works great but you have no physical audio output to monitor/hear sound, so I thought I could enable internal audio IF (as I've done on other devices) and that will be the audio output instead of needing another second IF to get audio from.

So I decided to rescue some past investigations I made when first researching about RPi, along with the great content generated by some dear great folks in here, and some new research one after each problem.