I have imported them into Media Manager (the PhatHack version, 0.9c?), and the tags seem to be parsed correctly - at least I do not see any "unknown"s showing up in the list of genres, artists or albums.
Unfortunately, when I ask Media Manager to recreate the DMS, I get a lot of blank entries when I play the DMS in the car. I assume these are equivalent to the "unknown artist" etc. when I have Music Manager create the prompts.
Before you eject the DMS, look in the PHTDATA partition. Somewhere in there, you should find a bunch of WAV files which are the announcements. Pick an album you know doesn't announce in the car and see if there's an WAV for it. If so, play it on the PC and see if it works properly.
I'm not familiar with PhatHack MM, but I do know that the PhatBox is picky about its files. That could be the problem.
If the WAV file isn't there, then PHMM apparently didn't generate it. As I said, I'm not familiar with that program, but the PhatNoise Managers tended to not generate announcements for music already on the DMS unless you specifically asked for it.
You could also try
PhatVoice which is a stand-alone announcement generator for the PhatBox. Be sure to select the "Always Replace Output Files" box - it will take longer, but will guarantee that all announcements are re-created.