If you click on the Custom

If you click on the Custom button, then the Audio tab, there is a checkbox that allows you to specify to include an audio track. When burning a DVD, DVD architect will re-encode the audio as AC3, which is why most folks will just make a second pass through the project to create a separate AC3 file, which DVDA will pick up if it’s available.

I have found that MPEG2 runs about 2GB/hour so your size seems correct.