Hi
I wouldn't think this was an intentional limitation on the free version as mine works OK. 
Have you tried saving your film first to your computer, and if so does it play?
Reload your project (Assuming you saved it.) Now just select:
Save Movie - Computer/Data - Browse for a destination folder - Preset HD 1080 - Encoder options - MP3(Native) - Sound 44100 Hz
The output Avi should play in Windows Media Player.
If this is the case then your film is OK and at least you have a playable copy on your PC.
Try saving again to disc with VP and if the sound is still absent I would suggest using a 3rd party burner to create your DVD from the saved avi, e.g. Windows DVD creator.
Nat