I can recommend you to check the following:
Make sure the tracks are not muted.
Try to play back the files using a different media player to confirm they work.
Try to load a file recorded using a different source or different format to see if the issue may be related to those particular files.
In case you still have issues, you may reinstall the program using this link: https://www.nch.com.au/mixpad/mpsetup.exe