Steps
1. Left Click "Options" on Sony Vegas Pro
2. Press shift key + Click Preferences (If directly click preferences, you will not get Internal tab option)
3. On the tabs select "Internal".
4. On the bottom of that window you will get a search box "Show only prefs containing" write main concept...
5. By default MainConcept AVC/AAC is set to "FALSE" change to "TRUE"
6. Hit Enter, click "Apply" and then "Ok"
You will be good to go.
