OK this is a turnup for the books, I thought no-one would care.
What I did was repoint the soundfile so what it thought was mario was actually the "Tetris classic" song (go to the music test menu if you want a further demo). I could have sworn it was called the Kalinka though:
http://en.wikipedia.org/wiki/Kalinka (I know it is Wikipedia but check out the demo song at the end).
Yes it will work on wifi (the length of the file and the filesystem is untouched).
As for a US version, the US and Japanese version have the same soundfile except they are made up to 4megs by having 00's added to the end (the Eu version is 1.68megs or something).
I will go make a patch but if you patched the Eu version dump the soundfile from the patched version, make it up to 4megs with 00's (I would personally just make it over 4megs and use GBATA's fix overdump) and finally re-import it will work fine.
If you want a mario hack, compared the differences in the patched version to get the place you have to patch find the offset of the mario file (I used the nice info made by SDATTOOL when I extracted it:
http://djbouche.net/wp/sdat-stuff/ ) then do a "32bit unsigned long" byte flip on the offset (to make it DS compatible) and copy that over the repeated section in the patched file. (I will try and make it sound a bit easier but that is from memory).
As for the custom soundtrack the music files are SSEQ files (a very common DS sound format, others are BANK and SWAR if I recall) so you could probably insert and repoint the sound file (I would use a US/J version and add the new SSEQ into the 00's and repoint or you could use NDSTool and rebuild the filesystem (it would screw up some patchers though)).
At this time I do not know how to turn windows midi into SSEQ (I am working on it) but you could easily use anther sseq file from another rom (I sense a Phoenix Wright tetris coming up).
Consult the DS soundfiles reverse engineered thread or my upcoming final release of GBA/DS rom hacking 101 (I left out the DS soundfile hacking section as it was not finished (I needed to do far more testing) for the taster release).
Edit:
@omegatr0n the music I repointed to is the music you hear in the final few levels of the marathon mode (at least before you go to endless).
As for a "hard mode" hack, if you are saying level 5 against the CPU is too easy (I can beat it but I will be damned if I can do it more than 4 of 10 matches on a good day) then I bow to your Tetris mastery. A true hard mode hack would require someone to tweak the AI which is ever so slightly above me (ever so sllightly = about 5 years intensive learning).
