I too vaguely remember someone describing this problem on the DTVPal thread. IIRC he'd recorded some PBS shows on his Pal, but it would only play back the secondary track, which was either Spanish, descriptive audio, or just silent (I don't remember for sure).
Since the Pal records all data for a subchannel, both soundtracks must have been "in there" somewhere, but I don't think we ever figured out a way to play the primary track. (The Pal's proprietary file system often has its drawbacks.) Our best suggestion was to mute the sound and turn on subtitles
I've never had this problem myself during playback, but it did happen to me - once - during live viewing. Changing the channel and changing back fixed it. Maybe a glitch at the start of viewing/recording can "fool" the Pal into thinking the primary track doesn't exist, or that both tracks are flagged English (which makes it impossible to choose between them).
Since there's no apparent pattern to the problem, is their any indication that reception is weak/marginal, such as pixelation/breakups, etc. on the Pals showing the problem? (I wouldn't go by the Pal's signal strength meter - the signal may be strong but shifting multipath interference can still cause reception problems.) If so, a more directional TV antenna might be an easy fix.
One defense might be to record the same show twice, starting one recording a minute later than the other. (You could set one recording from the guide and the other manually, VCR-style.) If a reception glitch is causing the problem, it's unlikely to affect the start of both recordings.