Missing some applause from about 10 seconds of a project.
How can I render just that ten seconds of sound and update the .avi file and thus avoid rendering the entire project?
I think you will have to render the whole project.
If you render to avi, you could use that file and just add the 10 secs of audio and that way should be faster. You wouldn't need to render multiple tracks or FX again.
Re-render is 11 hours.
Rendering that ten second clip as an .avi file and and adding it to the existing .avi file, resaving and dragging it into DVD-A, right?
If this is a DV AVI file then you can use smart-render. Make a timeline selection of the 10 seconds you need. Go into File | Render As... and make sure Render loop region only is checked. It will just render the 10 seconds. Now open a new project and place the original render and the new 10 seconds back to back on the timeline and select File | Render As... again. Vegas will see that both clips are already in the proper format and it will not re-render but rather intelligently copy the files into a new file with both parts (this is called smart-render). Use this new AVI file in DVDArch.