Bluetooth devices utilize different profiles to deliver audio to BTunes. While playing music and audio the transmitter will use the A2DP profile (Advanced Audio Delivery Profile) to send high quality audio in stereo to the BTunes. However, during a call or when the microphone is used on BTunes the transmitter will switch to the HFP (Hands Free Profile). This profile has significantly lower quality than the A2DP profile and only sends mono audio but allows the pickup of voice from the microphone. This is why during a phone call the audio quality on the BTunes will drop, this is because the device sending the audio signal is sending a signal with poorer quality than normal. This is currently the highest quality bluetooth profile for use during a call and is not something we can change. After the call is over then the audio profile should switch back to the A2DP profile and audio quality should increase again.
In certain games that allows voice chat the computer will switch over to the HFP profile to allow the microphone of BTunes to be used. Sometimes this will only switch over when the push-to-talk key is pressed or it may be always on, this depends on the program being used. One possible way to work around this drop in sound quality is to change the voice input device to another separate microphone or disable voice chat completely.