I think
https://zoom.us/test offers you to test your setup without actually joining a meeting. The test session lasts only for about a minute or so and is pretty annoying, but in contrast to an actual session, you hear what you are producing. The one thing you need to be careful of is that this test session sets up your video and audio and then states "everything is working" with a popup window with the single button "End Test". Don't press that button but ignore the popup:
this is the echo setup you need for actually checking your audio quality. So play your accordion and check what you are hearing back in response. While the session is on, you can actually click the hatchet next to your microphone or camera symbol and change settings around (like your audio source and the infamous "original sound for musicians" check mark, assuming you have it). As long as you leave the "End Test" popup alone