How can i change the volume in A2dP source mode

[sourxsunny]

the play_bt_source_example not show how to control the volume .

my test A2dP sink device has no controls, so it is not possible to change the volume on it

[sourxsunny]

can i use alc to control the volume ?
[sdcard]-->fatfs_stream-->mp3_decoder-->-->alc-->bt_stream-->[bt sink]
but i don't know how to make it work.