Indeed that is an odd one. There are plenty of threads in this section that would have done, to say nothing of the member of the original post "was last seen:
Mar 25, 2017 ".
Oh well new thread made now.
We are going to need a game name to be of much help here.
As a general rule undubs regardless of whether they use the common SDAT formats or another format (
https://gbatemp.net/threads/the-various-audio-formats-of-the-ds.305167/ ) tend to a matter of finding the audio you want in your game in the other region (usually Japanese), overwriting the ones from the region you want to play and rebuilding the ROM with this overwritten audio.
There can be things that get in the way -- sometimes a game might lose some audio between languages (games dubbed in Japan might not be outside it as dubbing sessions are expensive), other times it might pick some extra up, sometimes you might only want a partial undub , sometimes the game still includes it but needs to activate it (Castlevania had one of these, though if you had been really really bored you could still overwrite individual sections, sometimes the audio can be normal but packed into another file (I think it might have been one of the Touch Detective games I looked at that did this) though this is rare indeed as audio is usually the one thing left outside, sometimes the localisation actually did a lot of stuff to the game (audio games are probably the common for this one but I have seen plenty of other things too), and sometimes it is a minor tweak but one still prevents the overwrite and rebuild approach.
.farc is not a common format extension (I am guessing it is short for file archive or something, especially if there are a lot of them). Tinke or one of the other big all in one programs might be able to make sense of it but don't count on it.