1. Go to Settings -> Devices -> Bluetooth and Other Devices
2. Check that AVENTHO 300 appears under "Audio" and that there is a headset icon in front of it (headphones with microphone). Also, make sure that "Connected voice, music" is displayed.
3. If not, click on the device, then click on "Remove device"
4. Pair and re-connect AVENTHO 300 as described in the user manual
5. Go to Settings -> System -> Sound
6. Check that AVENTHO 300 is selected as the audio output and input device