Bluetooth transmits signals over a short range of radio frequencies, so it's not surprising that the sound effect is sometimes a little vague. The solution is as follows:

To test or improve sound
1. Place two equipment within 30 feet or closer;
2. Keep the two devices away from other Bluetooth devices, microwave ovens, wireless routers and other electronic products;
3. Make sure the Bluetooth headset is charged and turned on;
4. Make sure the device has enabled Bluetooth settings;
5. Turn up the volume of the Bluetooth headset and the paired device.