I think I've found a breakthrough on undubbing the CG cutscenes in the NTSC-U version of Metroid Other M, as I haven't seen it mentioned anywhere and no similar answers are found in Google search.
I've found some software called TMPGEnc that was able to unmux Other M's SFD files,
and I
CONFIRM that the SFD's in the USA retail release have
BOTH English and Japanese audio tracks!
I posted my original findings in ShadowOne333's thread for his
Metroid: Other M Redux project, as I used his methods for editing SFD files as a guide. He also helped me out immensely with getting my edits to work in-game.
https://gbatemp.net/threads/metroid-other-m-maxximum-edition.602301/page-8
There is indeed some kind of switch in the game's coding that detects and selects the English track. But since there is no option in-game to select the Japanese track, it's presence has gone unnoticed. However I'm 100% sure that the SFD files in both the Japanese ISO and the USA ISO is exactly the same.
On a related note, while I figured out after using Shadow's guides how to replace the video files, I could only get the sound to work if the file has
both audio tracks together instead of only one. Might be interesting to see if Shadow's edits or the Maxximum Edition are the same way.
So what needs to be done is to extract all the SFD files that use their own audio tracks in-game (Important: Many of the SFD's
don't have audio and/or voice tracks - they are actually intercut with in-game cutscenes with the audio actually coming from the AIX files instead.),
then use TMPGEnc to split apart the SFD files into individual files for each track - creating one m1v file for the video, and two ADX files for each audio track, and convert the m1v file back into an mpeg-1 file.
I then use
CryTools-win-x64 to mux the files back together like so:
"sfdmux.exe [final video].sfd [video file].mpg [Japanese track].adx [English track].adx" - in
exactly this order.
This will essentially flip the order in which the audio tracks are arranged so that the game is tricked into selecting the Japanese track instead of the English track!
I then created a Riivolution Patch based off Nintendo Maniac's XML script in Shadow's thread that replaces the CG cutscenes with my new SFD files.
There could possibly be some kind of switch in the game's code that can be flipped to play the Japanese audio tracks that we could create a hack for instead, and I can't speak for trying this on actual Wii/Wii U hardware.
But for playing the Japanese CG cutscenes in the NTSC-U version on Dolphin, I think this settles it!
You don't even need a Japanese ISO to do this - Everything we need is all on the USA ISO!
Edit 7/5/22:
The official download for the freeware version of TMPGEnc is here:
https://www.tmpgenc.net/en/download.html
After doing some more testing, there are some caveats to rendering and converting with TMPGEnc.
For files that won't easily cooperate, I recommend using
ffmpeg to convert the video after you've extracted the audio from the SFD's using TMPGEnc.