ok, I think I figured out.

Once I opened the bluetooth SCO, it not the bluetooth headset entering hnads-free mode, but instead is the android system that mute all streams other than voice call stream.

To fix it, just use `audioManager.mode = AudioManager.MODE_NORMAL` after SCO connected. This will turn system back into normal mode, aka routing all streams including music and voice call to the bluetooth headset using SCO protocol, allowing the app to fetch mic input from bt headset.

The android document didn't mention this at all. The ChatGPT said this will won't work and suggest that I use AudioFocus. LOL