Okay, so with Memphiz's kind help, we figured out the issue, which of course was an ID 10 T error on the user's part
For those newbies who are as stupid as I am, here was the issue:
We found that my mp3 files were playing back fine in File mode, but not in Library mode. Why? Because the ATV2 "official" build that I'd installed is not the same "stable" build as XBMC for Mac, which I'd been running on my laptop, and guess what--you can't just copy the databases from one to another. The ATV2 official build is based on the nightly OSX build, not the stable OSX build.
So, the solution:
1) Export the existing OSX XBMC libraries as a backup.
2) Upgrade my OSX XBMC version from the "stable" build to the nightly build, and hope the databases are upgraded automatically.
3) If the database upgrade fails, then import the backed up libraries from step 1.
4) Upgrade my ATV2 XBMC to the latest nightly build.
5) Copy the databases over from my OSX nightly installation to my ATV2 nightly installation.
Hope this helps anyone as clueless as I was!