Just as a follow up after the AirPods Pro Suck... I've been listening to music today (Apple Music, mostly chill electronica), and I'm delighted to have the original quality audio back. The bass and definition are much improved in ANC and Transparency modes. Funny how it just had slowly degraded until I noticed yesterday.
As an experiment, I was able to reproduce the problem by simply covering the inner mic grills with scotch tape. Results: less bass, bone conduction noise, less effective ANC, and failure to pass the Fit Test. Likewise, moisture on the inner mic grills causes the same effect.
Seems like there should be a solution. Perhaps a software update could learn to compensate around the changing inner mic response over time, and at some point if it reaches a limit, suggest a cleaning. Preventing the problem from happening would seem to require at least some kind of oleophobic coating on the mic grid (maybe there already is one), or replacing the mic grid altogether with a flush membrane and moving the vent elsewhere.