Does the audio vanish in preview, or just at export? What version of VP is being used?
Should never request the code once it's entered.. Try this 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 it continues to prompt, get in touch with support... https://www.nch.com.au/support/reg.html