First, under OPTIONS | DISK tab, Clear Unused Cache Files.
Please open the Windows Volume Mixer. Right-click on the 'speaker' icon at the right-bottom of the screen.
Confirm that the Videopad volume slider is at a normal level. 
Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite.
Last ditch effort, share your project. Someone here will find an answer. It's easy, quick and can be private. Just follow these steps carefully...
Back up --- With your project on the timeline, click on MENU at the top-left. Click FILE|BACK UP PROJECT FILES TO FOLDER. Choose a folder and SELECT FOLDER.
Upload --- Use a free server - Google Drive, MS OneDrive, etc.* - to upload the saved, numbered FOLDER. Do NOT upload the individual VPJ or export file.
Get link --- Get a public link. If using Google Drive click GET SHAREABLE LINK. Change "restricted" to "anyone with the link can view" Click COPY LINK | DONE.
Share --- Paste that link here, or click the folder at the top-right of this forum to message it privately to me. It won't be shared.
* Before uploading, right-click the folder, click PROPERTIES. Look at the File Size to confirm that it's not too big for the free space on the server.