Are the voices played in the Classic release of the game?
Did you check/change the voice sliders in the audio section of the options?
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.
Check the configuration of your speakers in Windows. This problem can be caused by having stereo speakers set to 5.1 sound, for example.
- Right click the Volume Control speaker icon in the taskbar and select 'Playback devices'
- In the Playback tab, right click the 'Default Device' (with a green checkmark) and select 'Configure Speakers'
- Make sure the 'Audio channels' configuration matches your speaker/headphone setup; press the 'Test' button to hear sound effects played on each speaker, while the corresponding speaker in the 'Speaker Setup' window is highlighted. You can also click directly on the speakers in the picture to hear the test sound effect on that speaker only.
You could also try changing the Windows sound quality settings (one person with D:OS EE reported that a very high default sound quality caused performance issues in-game).
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?
If you are using speakers, do you have headphones you can try, or vise versa?
Check for updated drivers, and Windows updates in general.
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 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.