Something unusual is going on and it has me perplexed?

I have a 51 minute project. When rendering for DVD, the mp2 renders normally. After I change render settings to render the AC3 file, Vegas renders the first 31 minutes normally, then blank audio for the remainder of the file. Also, it takes the full 51 minutes to complete the AC3 render, a task that should be 6 to 8 minutes.

The complete audio is present on the timelime, with no filters. Levels at 100%

Any ideas?

Thanks