There are a few things that could be causing your Bluetooth audio to not work in a GSI ROM. The first thing you should check is whether or not your Bluetooth adapter is properly paired with your device.
If it is, then the next thing you need to change 2 things in Phh Treble Settings. Follow this step,
- Go to Phh Treble Settings, go to Samsung features > Set eSCO transport unit > Choose 16.
- In Phh Treble Settings, go to Misc features > Force disable A2DP offload.
- Then reboot.
After done this your Bluetooth audio should be fixed. If you have any question about GSI ROM related join our telegram group.