Sound hacking has been well covered in the last few days but most DS roms use the SDAT format for sounds. It originally stems from the days of GBA flash carts where they only had perhaps 64-128 megabytes of space if they were lucky and you would go and replace the sound with one of the ones from goldeneye or I think it was a kirby game to drop the size (often by quite a bit).
Pull apart the rom at a file system level and find the SDATs you want. See rom ripping and enhancements link in my signature for how to or look up DSLazy/DSBuff and I seem to recall treeki made a tool considered to be even better than ndstool (which the link in my signature advocates and DSLazy and DSBuff use).
Replace US version with Japanese or whatever you want.
Rebuild rom (most tools that can pull it apart can rebuild as well)
Problems come in the the US sounds and Japanese sounds do not match properly (as is the case in this in rockman ZX/megaman ZX). Sometimes you will just get an odd sound for the occasion or no sound at all but other times (and this is one of those) the game will lock up.
This means you have to tweak the Japanese sound file to match the US one: same ordinals and names for respective sounds as either can be used for all the files (and there are normally about 60 of them).
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