BM70/71 implement A2DP audio source
The BM70 chip is a Bluetooth Low Energy (BLE) module.  As far as I know, BLE is not meant for streaming applications. These posts suggest that A2DP and BLE are not compatible:
https://stackoverflow.com/questions/22141569/how-are-you-able-to-get-an-a2dp-bluetooth-profile-to-work-using-bluetooth-4-0-b
Does Bluetooth LE stack supports A2DP profile?