Try deleting preferences: hold down command and option while starting and click Delete Preferences.
Also - in the export settings, make sure that both video and audio are selected.
What are the clip audio format and sample rates? What are the project settings?
There have been several similar reports recently, but I don't think that a clear cause for this problem has emerged yet.
One other thing you may try: create a new project, make sure its settings are as you intend (don't leave them at automatic), select all from the current project, paste into the new one, try exporting that.