You may experience the same problem that: I've just updated to Windows 10 today, everything works wonderfully so far EXCEPT I've realized the sound is super low.
So how to fix it? The collective methods below could be the most possible solutions to the problem.
1.Enable “loudness equalization” (Control Panel-Sound-Playback Device-Enhancement).
2.Switch to ”Communications” tab under "Sound", make sure ”Do nothing” is selected.
3.Update/uninstall the audio driver.
Update: Device Manager-Sound, Video, and Game Controllers-Right click Realtek- Select update driver software-Browse computer for driver-Select from the list of drivers-Chose HD Audio Driver (do not choose Realtek).
Uninstall Right click Realtek- Uninstall-Restart computer.
4.“Setting”-Turn “Hey Cortana” off.
5.“Control Panel”-”Power Options”-”Check High Performance”.
If you hear low-level sound through headphone or speaker on your PC, the issue could be with only your front or back audio plug, which means one of the jack is still working normally. So just try to change an audio jack.
If you would like to suggest any other methods or you have solved the problem in your own way, feel free to comment below!