Hmm theoretically, you could spin up a second Immich and mount the folders in there as external libraries with write permissions 🤔
This is just an idea which came to mind reading your post. But I would assume that messes with the metadata stored in your primary Immich instance or rather its DB. So probably not the best idea in hindsight. Also even easier approach to achieve this would be using a simple imager viewer tool directly on the filesystem 😅
Well besides the excellent options already mentioned by others, there is also Finamp, a Jellyfin music player.
Others have also mentioned Symfonium which can play audio from a Jellyfin server as well, not only from a subsonic one like Navidrome.