Normally we would require much more information from you before we would be confident in giving you the correct diagnosis or a possible explanation but I will try on the following assumptions.
That you used MEP (Movie Edit Pro) with Windows 7 or 8.
That you are now running Windows 10 and have updated your software to the 2021 version.
That you are using your computers audio and it does had the 'Stereo mix' section within it's architecture.
It is the Stereo Mix part of your inboard sound chip that is important. It is that part which is disabled by Windows 10 by default and need activating.
Go to settings \ System \ Sound \ Sound control Panel and open it.
In the recording section make sure Stereo mix is enabled and apply.
Also make sure that the Microphone RealTek (R) Audio is set to be the default device.
Then open the Microphone privacy settings and make sure that were it says 'Allow apps to access your microphone' is set to on.
. . . . Movie Edit Pro Plus - I get no sound during playback. . . . .
Do you you mean when playing back the timeline?
We do need more information:
computer specification - see here for details required, and Windows version and build number - please quote processor and graphics card make/model in full.
the version number of Movie Edit Pro Plus as found under Help, About
a screenshot of the Program settings, Playback and Video/Audio tabs
do you here any sound when playing a video clip outside of Movie Edit Pro - eg in Windows Media Player?