Two muting locations to verify - the track ... and the clip. 
Also, right-click on the audio clip and toggle the MUTE/UNMUTE choice. And, as Nat says, be sure that the volume slider is at the far right.
If neither works, back up your project and then close Videopad. Reopen it and before loading the project click OPTIONS | DISK tab and Clear Unused Cache Files.
If still no joy, share the project. Just follow these steps...
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. If necessary change "restricted" to "anyone with the link can view"
Share --- Click COPY LINK | DONE. 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.