UPDATE:
(1) I found that though I’d made the drive (/mnt/pmsmedia) pi:pi, for some reason, the permissions for the owner were --x. (So glad for the ACL stat command!) That is now fixed and I can once again read and right.
(2) Thanks @drmagoo. Somehow my mp3s were corrupted. So I’ve re-ripped a couple hundred CDs. And I’ve found even more backups of my mp3s in an old Dropbox. All are now loaded on me PMS and are streaming happily.
Thanks so much for the help!