@TrekkiesUnite118
In regards to the voices, we're on the same page. My previous post was based on my findings from digging around in PowerISO, so I'm aware of the Saturn/Original version's in-game (meaning non-FMV) radio chatter audio being stored as .CDA files separate from the main BGM, meaning that new dubbed audio .CDA files can be "dropped in" without affecting the main BGM.
As for the "radio chatter" .CDA files that also feature baked-in SFX/music, as in the previously cited example of the one-minute survival portion of the final boss, those will be dealt with on a case-by-case basis.
I never meant to imply that I believed that the in-game "radio chatter" voice over tracks (including ones that also contain incidental music/SFX) were baked into the main BGM during gameplay. I must have phrased some things poorly and I'm sorry for the confusion.
I assume this is too obvious to be mentioned, but to make sure I'm not misunderstood, the audio for the intro and outro FMV will require a different approach, seeing as all audio (music, SFX, voices) are obviously baked into the .CPK files as a single audio track. So in order to do a new dub for the FMV specifically, the .CPK audio tracks will need to be reconstructed using stock sound effects, music directly from the OST, and, of course, the newly-dubbed voice overs.
I can see the confusion about re-subtitling the intro and outro FMV. I didn't write out my thinking there, and also had a misconception that I wouldn't be able to directly edit/replace the audio tracks in the source .CPK files from the disc. For some reason, I thought I would need to convert the .CPK file into something else more common like a .AVI in order to swap out the audio track then re-encode it as a .CPK, hence my worries about further compressing it. I'm glad you said that -- I guess I had a misconception that .CPK files were super proprietary and couldn't easily have the audio track swapped out without needing to be re-encoded. Thank you for clearing that misconception up.
As for the subtitles themselves, since I was previously under the false impression that I'd need to extract and re-encode the FMV .CPK files anyway, that I should do a re-sub as a way to become familiar with the .CPK encoding process and also have a nice proof of concept and an option to have both a dubbed release and a subbed/dubbed release down the line. If memory serves me correctly, you were able to achieve higher quality FMV than the Saturn original in your Lunar: SSS translation by using a higher definition source and re-encoding it. I also thought that could be a perk, with or without new subtitles.
So, thank you for making me realize that it's not strictly necessary, though I may still consider creating a new .CPK from the HD XBLA source and a subtitled FMV ISO option as potential QOL improvements.