The media adapter is apparently upgradeable, with one patch already planned...

A few days ago, Nintendo of Japan updated the official Play-Yan website with an announcement detailing plans to release a patch in July that would make the Play-Yan compatible with full MP4 audio playback (essentially enabling stereo sound in movies.) The Play-Yan, a first party multimedia playback device compatible with GBA and DS, initially supported only mono sound in its movie playback.

The process will involve simply downloading the 500KB patch and making sure that it is present on the SD media card root directory. The Play-Yan device would then read it automatically at startup. Apparently the device's firmware itself is not altered, and the patch must be present on the SD card whenever MP4 playback is desired.

Regardless, it is very cool to see the device being supported by Nintendo with additional functionality. Hopefully the efforts will continue (Nintendo is also planning on following up with new encoding software that will allow users to allow for MP4 encoding) as the peripheral sees expanded distribution in Europe (US release currently under deliberation) later in the year alongside the new GBA Micro.

UPDATE: Nintendo now has the patch available for download at their official Play-Yan website. Again, this patch enables MPEG4 playback capability, the same as the PSP uses (and which allows full stereo audio playback.) Any early adopter C-E readers who went for this nifty peripheral?