Is the volume slider to the left of the track fully to the right?
Is the track muted at the left with a red X through the speaker icon?
Is the clip muted? Right-click on its audio track to check.
Are you using version 8.56? If so, install this one instead. 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 a problem please share your project. Follow the instructions here to upload a PORTABLE project (not the export). Be sure to share it (make it public), non-restricted...
http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
If it contains sensitive material link it in a personal message (PM). Click the envelope above. It won't be passed on or retained. Use FILE|SAVE PORTABLE PROJECT AS, not simply SAVE PROJECT. Someone here will check things out.