It may be due to your DG40S is operating in Bluetooth handsfree profile while in call mode.
If you use plug & play:
Step 1 Find and right click the Speakers (Sound) on the taskbar;
Step 2 Choose "Playback devices"
Step 3 Find "Communications" and choose "Mute all other sounds" which shown as below image.
If you use Bluesoleil software:
Please right click the music device icon and click "Mute on" to turn off the Bluetooth handsfree profile.
like below image -