Help "List of audio devices changed" pops up and turns off recording from cassette player?

Generally the thing that causes that message to turn up on any machine is active socket sensing, and how well or accurate that is rather depends upon what you've plugged into it at any time. But the weird thing here is that this doesn't apply directly to USB devices as a rule (although I suppose it might be different on Macs, but I don't have one). But I suppose that it's quite possible that on a device that aggregates inputs (that's your Mac) that if anything connected to any of the aggregated inputs changes, then that message would flag up. Even unplugging a pair of headphones in a shared socket could do it...