If you intend to adhere to some sort of standard, then 24 KHz MP3's would probably be your best bet.
If you're compressing them into a format that allows custom sample rates, then go for 28837 Hz.
Edit: Re-read the question. Max. sample rate for a PAL Miggy is 28837 AFAIR. I haven't tried to play such files in Hippoplayer, but the sampling program I used back in the days (Perfect sound) had no problems playing such files.
