No DRM issue. All music was ripped from CD.
Anyway, I think I've fixed the issue. I reformatted the card, which did nothing. Then I tried deleting everything via DM>Device options> media and reloading it. Presto, it worked.
I think DM had a residual copy of the old lower bitrate mp3 version somewhere on my Mac (before I re-ripped) and wanted to keep loading that onto the BB? When I re-ripped, it somehow confused the BB as to how to read the tag, having a residual mp3 version but being told it should be AAC, or something like that.
Once I deleted all media via DM (instead of just formatting the card) those residual copies that DM was using were deleted as well, not just the copies on the SD card.
That's my theory anyway.
In any case, problem solved. Thanks for the reply!