No Volume, No Visual Level

Please help me, I am at my wits end.

I'm on Logic 10.4.8 on Catalina. I've been using it with no problems.

Today, I have lost all sound in Logic in playback or playing virtual instruments. Old sequences that used to work, nothing. New sequences that I create, nothing.

I cannot hear any level, and no level appears visually in any channel strip. When I play virtual instruments, I see MIDI being triggered, but see no level and hear no level.

I have:

- doublechecked the output in the audio preferences - no output works - no change no matter what the output, headphones or speakers

- pulled all plugins out of components - no change

- trashed logic preferences and restarted - no change

- unchecked and re-checked core audio - no change

Please help me. I am going crazy, and I have a project due in about 16 hours from now.

Thank you.

OK, the only thing I've noticed here is that I get a System Overload message with the advice to increase the buffer, every time I open Logic. I have increased the buffer as much as I can with no change.

Can anyone help?

OK, I'm going to keep posting here, in case someone else eventually stumbles across this with the same problem. It is the weirdest thing I've ever seen in computer audio.

Here's the full story:

Yesterday, I was using Logic to run an audio sequence while I was recording myself through a movie recording simultaneously through Quicktime. I was using my daughters Airpods to monitor the signal, although I was not using the Airpods to record my voice. Instead, I was relying on my built-in MacBook Pro microphone. The Airpods were weird and flaky throughout the process, but I managed to push through as best as I could.

After I finished recording the video, I closed Logic and opened iMovie to edit the video I had recorded. I edited the video and then reopened Logic. That's when the conditions that I described in the first post began. No audible sound or visual level in any channel in Logic, no matter what I tried. Old sequences or brand new sequences - nothing worked. I tried all of the remedies listed above with no change.

So, the first step to figuring out what happened was that, after many hours of struggling, I plugged in my my Blue Yeti microphone, and discovered that I was able to hear sound through it from all channels in Logic, and could record into it as well. With a little experimentation, I've discovered that when I set the audio input setting on any output, it was fine. BUT, if I set the input to Macbook internal microphone, everything immediately no longer works.

I opened Zoom to see what the effects are there, and unless the Yeti is set for both output and input, then no sound works.

So I think what I'm coming to understand is that I need to reset the sound preferences for the entire computer. Does anyone know how to reset those preferences?