Why do Bluetooth headsets use music to send and receive messages?

Because under normal circumstances, it is normal for you to make a phone call or send a message even with a Bluetooth headset or a wired headset, because you only connect Bluetooth for the sound or sound that the headset can't hear when ringing the phone, so it is normal under normal circumstances.

Check whether the Bluetooth headset is connected to my mobile phone. If you connect to someone else's mobile phone, there will be sound series.