If you didn't already do so, try verifying local files: in Galaxy select Divinity: Original Sin 2 in the left column, then click on the More button, and in the Manage menu select 'Verify / Repair'.

Try changing the audio volume in the game options, and maybe try changing the audio quality.
Try changing the Windows sound quality settings (right click the volume icon in the task bar, select Sounds, in the Playback tab right click the default device and select Properties, then switch to the Advanced tab, etc).

Are you shutting down all non-essential programs before starting the game?
With D:OS EE a couple of people had sound problems with the Razer Synapse program running in the background (one with Razer Kraken headphones had it cut out completely after a few minutes, while another with a Razer keyboard and mouse was getting sound stuttering). I'm not sure that could account for a difference in the DE vs Classic for D:OS 2, though.

Try starting the game directly from the '..\Divinity Original Sin 2\DefEd\bin\EoCApp.exe' program file, by right clicking and running as administrator.
Try right clicking the executable, select Properties, switch to the Compatibility tab and check off the 'Override high DPI scaling behavior' option (set it to Application), maybe run as administrator and disable fullscreen optimization, and set a Win 7 or 8 compatibility mode.

Have you tried lowering the graphics settings and resolution, or switching to Windowed or Fake Fullscreen display mode, to see if that would make a difference? Windowed mode should at least be more convenient while troubleshooting.

Try doing a clean boot and then test the game. Click Start, or hit WinKey-R, type in msconfig and hit enter; in the General tab, click Selective Startup, uncheck Load startup items (if required) and leave Load system services and Use original boot configuration options checked. Next, click on the Services tab, check the box to Hide all Microsoft services, then click the Disable All button (maybe make a note of which are currently enabled/disabled), then click OK and reboot the computer.
Run msconfig again to switch back to the normal boot configuration.

Try creating a new Windows administrator user account, switch to that account and try starting the game from there, directly from the executable to test.

Check for updated drivers, and Windows updates in general.