https://github.com/KieronQuinn/AmbientMusicMod seems to be licensed GPL-3.0. For that reason, it could never actually be bundled into GrapheneOS as the project doesn't want to include things without a permissive license.
Even if that wasn't the case, it's probably not the kind of thing that GrapheneOS would add to the OS. Don't get me wrong, the feature sounds now, and I wish we could get the proper Google implementation that works offline, but it needs privileges that GrapheneOS is not willing to give it.
At the end of the day, while GrapheneOS does its best to ensure that the user experience is as good as it can be, it also has to focus on its core principles, which are security and privacy.
All of the current ways to implement this/make this work seem to go counter to that.