I stumbled upon this issue earlier this week and only found out what the problem was moments ago. I was unable to find someone else who had reported this issue so I submitted a ticket just now and decided to post here as well in case someone else was as stumped by this issue as I was.
TL;DR: If you play MCC on both PC and Xbox; lowering your Audio settings on PC will cause Halo 2 Classic (only!) to be affected by these changes. If you are experiencing that Halo 2 Classic is quieter than you feel it should be, double-check your volume sliders on MCC on PC.
This is a slightly edited version of my ticket to 343i, includes repro steps at the bottom:
> This issue is a problem between BOTH Microsoft Store PC MCC (don’t know if it affects Steam version) and Xbox MCC.
> It ONLY affects Halo 2 Classic, but it affects in both Campaign and Multiplayer, both during the game and during cutscenes.
>
> This was a bit of a confusing one to figure out but I’ve finally located the issue and am able to reproduce it.
>
> I play MCC on both PC and Xbox; on PC because it is my preferred platform and I’ve loved the game so far on there, but I need the Xbox version still to play Halo 2 and later as well as to play with my friends since cross-play is not a thing yet.
> It’s not too uncommon for me to lower my audio settings while playing due to being in calls with friends on Discord or Xbox Live Parties through the PC app.
>
> A few days ago while playing on Xbox, I noticed that my Halo 2 Classic games were nearly completely silent when I played. This was confusing since there are no volume sliders on Xbox (except the Main Menu volume) and also, all other games were fine. Even H2A.
>
> I tried all the usual troubleshooting steps: signing out and back in, power-cycling the console, even reinstalled Halo 2 content and finally ALL of MCC. Even tried doing a “Transfer Copy” from my other Xbox One system (which my roommate is using) since I knew everything worked fine when they were playing. Nothing solved the issue.
>
> I was just in the middle of recording the issue itself to submit here or post on the forums when I had one final realization; since the two games keep syncing data between them, perhaps the lowered audio is the cause? So I started up MCC on PC, went to audio settings and pulled the audio slider all the way to 100. Saved changes and exited the game.
>
> Booted up MCC on my Xbox and, as expected, it was syncing the latest data. I went into custom multiplayer and chose Halo 2 Classic. Slayer on Lockout just as a test and started the game.
> Audio fully clear, no issues whatsoever.
>
> Just to make sure I wasn’t crazy I did the entire thing again but lowering the volume back down to 0 on PC. Sync to Xbox and tried custom games in ALL titles (H:R, H:CE, H2C, H2A, H3, and H4). All of them still sounded fine, except, you guessed it, Halo 2 Classic, which was now dead silent.
> Repro Steps:
>
> 1. Launch MCC on PC.
> 2. Change Audio settings (i.e. from 100 to 0)
> 3. Save changes and exit the game.
> 4. Launch MCC on Xbox (using the same account). Data will be synced.
> 5. Launch any game of Halo: Reach, Halo: CE, Halo 2: Anniversary, Halo 3, or Halo 4. Audio should be fine.
> 6. Launch any game of Halo 2 Classic. Audio will be completely silent, as if the audio settings on the Xbox were set to 0. Menus still make normal audio noises.
> 7. Revert steps to fix the issue.
Not sure if anyone else have had this occur, but in case it did, I hope this helps.