What template do you have selected (the drop down that is directly under the Save as file type drop down in the Render As dialog box). You want to use the one that states "DVD Architect"... at the beginning. After rendering, render again using the ac3 file type, as the DVDA template renders an mpg that does not have audio in it. Make sure you do not change the name of the ac3 file, it has to be the same as the mpg file, other then the extension of course.

I believe the reason your recompress item is greyed out may be because you have selected a template that renders an mpg file with the audio included. DVDA must rerender this into one that has the audio separate. Since it must re-render anyway, the item is greyed out. If you use the DVDA template and separate ac3 audio DVDA will not rerender, unless of course your video, audio and menu media go over the 4.2G limit, then it will automatically re-render at a lower bitrate to compress the video to make it fit on the DVD, even if you used the DVDA template.