Maybe locked feature in free version, or are you pulling out the correct audio track off the DVD?
In any case, I stopped use RealPlayer a while back after they started loading up too much crap ware with installation.
Not a direct answer to your question, but you might want to try the free (and very lim) VLC Media Player from VideoLAN.
System is XPpro/SP2/IE7
I try to play DVD's in Real Player and I get brilliant video but absolutely no audio.
If I play ordinary CD's in my DVD player then audio is OK.
Equally I can play DVD's with full audio and video in Wndows Media and in Nero Showcase.
I have asked Real for help but got no reply. It is more of a nuisance than desparate but would appreciate comments.