I have bought a Bluetooth headset recently. After connecting the device, I was quite disappointed with the sound quality. Fortunately, after searching on the web, it seems that I am using the "Telphony Duplex" mode, which has low quality.

Here are the steps to switch to the mode with better sound quality:

1. Left click on the "Bluetooth icon" on the top right corner.
2. Move the cursor to your connected device.
3. Left click on "Sound Settings ..." menu item.
4. Change the mode from "Telphony Duplex (HSP/HFP)" to "High Fidelity Playback (A2DP)".

Reference: Ubuntu Forums: Bluetooth Headset Bad Sound Quality.