The poor sound quality may be due to the DG40S using the Bluetooth "Handsfree" profile which is more suitable for calls. Follow the steps below to switch to a more suitable profile for media (A2DP).
For Win10 (plug & play):
Step 1: Find and right-click the Speakers (Sound) on the Taskbar;
Step 2: Choose "Playback devices";
Step 3: Select the "Communications" tab and choose "Mute all other sounds" (see image below).
For Win7/8/XP (using BlueSoleil software):
lease right-click the music device icon and click "Mute on" to turn off the Bluetooth "Handsfree" profile (see image below).
This should switch the Bluetooth profile to A2DP which is much more suited to media streaming.