Did you burn the video into Video format or data format on Toast? Ideally you should have chosen Video -> DVD-Video.

When you drop the .avi file onto Toast's window, what video & audio format does it describe the video as having? Once dropping the video file onto Toast's window, have you tried clicking the Edit option & then the Video tab & pressing play to see if there is any audio?

You said VLC can play the video after burning. What about apple's DVD Player?