The filename_Audio.avi is an AVI file, not an audio file. But load it into Goldwave (the AVI), it will automatically extract and convert the ausio to WAV, then just Save the WAV file. Use that in TMPGenc.
Another very usefull tool for ASF file is ASFTools from
http://www.geocities.com/myasftools/ as it can do a ASF to DivX conversion as well as fix any file problems.