It would help if you could have elaborated on “older ballot station versions”, but yes, your PCMCIA card is not compatible with the ballot station you are using. It would probably be nice if the error message were a little friendlier than just “bad DLD”, which is a pretty meaningless error.
The problem isn’t really with MP3 vs PCM, as you can indeed mix and match with ballot station 4.3.x. The problem is that you are trying to put a 4..x memory card into a 4.3 machine.
I've got an R6 ballot station with 4.3.7 and CE 3.0 loaded. My older Alameda PCMCIA that works with older ballot station versions give me a "bad DLD" file error when I insert it. If I create a new ballot station with GEMS 1.18.10 and the new MP3 instruction audio, and redownload a PCMCIA, will the old audio work for the races and candidates? I guess the question could also be stated "can we combine PCM and MP3 formats"?