Sony Vegas mp3 problem

Hello , So basicly , long time ago when i bought Sony Vegas 11 i had problems with rendering movies.
The problem was that sound on timeline was perfect but when i render'd it it was like sound looping in the first 0,5 sec over whole movie. I found a solution for that long time ago , from what i remember i changed something into sony vegas configuration and everthing worked fine.
Now i had to reinstall Sony Vegas and problem is back...
I can't find the topic where i found the solution to that problem , and i can't remember what did u changed in the settings.

ANyone have any ideas , what could might be solution to my problem?

Best Regards