You load an mp4 onto the timeline and sound plays there for only three seconds? Does it play when the file is double-clicked in the BIN (top-left)?
Under OPTIONS | DISK tab, Clear Unused Cache Files. If that doesn't help, try the latest version. For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info.
If still no joy, upload the file in question and someone will check it out. Use a free server, such as Google Drive or MS OneDrive, get a shared (public) link, copy the link and paste it here or to me in a Private Message via the mail envelope in the top-right corner of this forum.
Using Google Drive, if necessary change "restricted" to "anyone with link can view."