Your mobile phone is probably (by default) set to HSP/HFP connection, which only supports low quality mono sound, or your mobile phone does not support the A2DP Bluetooth profile (see “What is a Bluetooth profile HSP, HFP, A2DP, AVRCP?“)
Solution: Set your mobile phone to Bluetooth Stereo (A2DP) connection.
Please refer to the instructions of your phone and/or contact the vendor.
