So I have used Android for a long time and all my phones, including my current google pixel 4a, have had their MAX volume limited: not enough to listen to classical music on the train.
----- 1st part
With custom ROMs, there is a simple XML in system/vendor/etc that you can edit (I had to fiddle with alsa commands on the phone, but it worked in the end). I followed a guide similar to the central part of this post
https://xdaforums.com/t/increase-speaker-headphones-volume-stable.4070111/
I don't think I can, but can I edit permanently files in system/vendor/etc or similar, with Graphene?
----- 2nd part
This app works
https://play.google.com/store/apps/details?id=com.goodev.volume.booster&hl=en_US&gl=US
I have happily used it for 1.5 years (first without GSF and then with Sandboxed GSF that I installed more or less when it was anounced) and it stopped working in August.
Since Aug, I noticed Google Play Store wants me to login, and this app just won't work if Google Play Store is installed without login. Previously I could see another page before the login, now as soon as I open Volume Booster, it jumps to Gplay login. Volume Booster works if Google Play Store is uninstalled completely; it seems it doesn't but the volume is increased. But that's not an option for me (I need it installed to pay parking, and basically only for that - what a wonderful world)
I tried to re-install the version of Google Play Store APK that I downloaded the time GrapheneOS introduced the sandboxed gsf, before Apps an its automatic updates; no luck with that too.
So without descending into the deep philosophy of Google Plays, any suggestion that would allow me to boost the gain of my headset without login to Google is appreciated. If you know how to do it in Java, I am happy to write my own APK for it! VolumeBooster is obfuscated (as anything now) so reversing it will take a long time...