Most localisations of Japanese games replicate the complete audio setup, and the overwhelming majority of DS games use the SDAT sound format. That typically makes undubbing of a DS game "open Japanese (or whatever version you want) of game, find SDAT, open other region version, insert SDAT, carry on with life". If a game has Japanese that is selectable/able to be activated within the game then a cheat ( http://gbatemp.net/threads/castlevania-portrait-of-ruin-us-undub-cheat.367014/ ) or mod might be made to force that to load but that is rare, similarly Japanese might be buried like unused audio tracks and you can then force that to load. Even if the game does not use SDAT then the solution tends to be the same (overwrite sound file with other file).
Occasionally games will be missing sections (voice work is expensive so they might not do it and just disable it in the other regions) and even other types of audio (I recall one with SDAT and ADX/AHX audio). I am always up for collecting examples of this but it is not common.
Equally some games might use SDAT but in a tricky way like including it in a secondary file system (most games doing such a thing leave the SDAT outside it, a few like Touch Detective do not though).
1 undub that is easy to make is for bleach dark souls. Also not a true undub but if you play the japanese versions of sonic rush and sonic rush adventure on a english nintendo ds system, the text is in english but the voices are in japanese.
Patches tend not to be released around here as it is still directly copyrighted works you are sharing via the patch. Legally speaking it is not really any different to linking up a rip of the audio, indeed as there are sdat players then yeah.
Things like the cheat above are OK, as might be patches to unlock it if it is buried in a game, and for more complex undubs someone might make a patch to extract one game, trim it down, modify what needs doing and insert it back into the host game before rebuilding it all as a workaround for the copyright issue.