DS games do not play MIDIs. MIDI is the industry standard file format for sequenced music, but DS games use other formats for this instead (such as SSEQ).

VGMTrans is able to convert many such formats to MIDI, but not all. Therefore, it's very possible that the game you're interested in uses a sequenced music format that VGMTrans doesn't support. Or maybe the game does use a supported format, but its music files are hidden in an unusual archive that VGMTrans doesn't know how to open. You'll have to find the music files yourself to check.

If you can find a SDAT file anywhere in the ROM, it's likely to be the latter case. If you can't, you should also try searching for "SDAT" in RAM while the game is running, in case it's hidden behind some sort of unknown compression in the ROM. If those both turn up negative, or you do find obvious music files that don't resemble SDAT or SSEQ, you're most likely out of luck unless you want to reverse-engineer the custom sequenced music format yourself.
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The Newer Team