embedded audio links *suddenly* stopped working

Hello all, I often embed dropbox-shared audio files (as well as other files) on my educational websites for students (copying the link, swapping in raw=1 instead of dl=0 at end, and inserting into suitable <iframe> part of html).

These embedded audio connections suddenly stopped working at some point in the past couple days. These are links that worked fine when I created them. I've done nothing to alter my website, but now the iframes render blankly (audio controls greyed out, no duration, title, or other data) as if the files don't exist.

If I try to take that link and paste it into the URL bar in my browser, I get a *flicker* of an mp3 player trying to load, and then:

.mp3 files are supported but something went wrong.

Oddly, my image and pdf files, all similarly taking advantage of dropbox-shared-link embedding, are still working fine. As I say, I'm convinced something has changed on the dropbox end, because I've changed nothing at my end. I found out about the problem from my students who were trying to do their assignments. 😐 

(Troubleshooting done so far: verified dropbox account is in good standing, verified no large files "banned" for causing bandwidth alarms, converted file from m4a to mp3, renamed, played around with swapping in every possible variation on the filename with and without dl=0, raw=1, omitting those altogether, etc. -- although, as I say, all of these are links that already did work fine. And analogous embedded links that are not audio seem not to be affected.)

Any insight?? Many thanks in advance!