Dell D630 can't play certain MP3 files through the speakers, but they play OK through headphones

[Werehatrack]

I have some MP3 files that were encoded by iTunes 10 on a different system, and they play through the normal output devices properly on all of the oter systems here, but on this D630 laptop, they play only through the headphone jack. All other MP3s play 100% normally. The subject MP3s play normally on any other system. This happens in iTunes, VLC, and Windows media Player. I've reinstalled the audio driver, upgraded the unit from 7 Home to 7 Pro, replaced the motherboard, tried alternate codecs, tried turning everything audio-related on and off, and gotten no change in the result. And I cannt even think of a way to make this happen intentionally. I;'m stumped. Anybody got a clue?

 

[Werehatrack]

I'm wondering if the files involved are mistakenly encoded with the audio in the rear channels of 5.1, but I haven't found a way to test that yet.