AC3 gives "Illegal MPEG audiostream" error in TMPGEnc

[rs008f]

I have a 5.1 AC3 audio file that gives "Illegal MPEG audiostream" error in TMPGEnc MPEG Tools. I'm trying to mux this with a MPEG-2 video file. I tried to convert it to a 16bit 5.1 wave using Besweet and then re-encode it to a 5.1 AC3 audio file, TMPGEnc will accept this file but the quality is much worse than the original. Below is the link where you can download the file to experiment with.
www.streamload.com/rs008f/initialbgm.ac3