We are missing some information here.
I must assume:
Windows 10*
Latest release of OpenToonz (1.2)*
16 GB*
If not that please advise of any differences.
I must also assume that you have heard sound play from inside OpenToonz.
In other words, we are troubleshooting Preview and Rendering but not basic use of audio files in timeline/xsheet.
One reason Preview or Render wouldn't play a sound would be that the sound icon is turned off in the viewer.
That should be on by default.
The icon can be hidden slightly to the right (at the bottom of the viewer) so you may have to scroll the menu over to check).
Another reason would be that you have rendered out to a format that does not support audio. For instance, if you render to a sequence of PNG files... PNG doesn't store any audio.
The primary formats used would likely be MP4 and AVI. Those should contain sound.
I'm sure this is something simple but we have to discover what that is.
*I have a long term plan to set up a Linux and Mac for use with OpenToonz but that hasn't happened yet.