C:\ffmpeg\ffmpeggui03>ffmpeg -i "C:\Media\Surround Test.wav" -ab 384 -ar 48000 -ac 6 -acodec ac3 -y "C:\Media\Surround Test.ac3"
Input #0, wav, from 'C:\Media\Surround Test.wav':
Duration: 00:14:43.5, bitrate: 4608 kb/s
Stream #0.0: Audio: 0xfffe, 48000 Hz, 5:1, 4608 kb/s
Output #0, ac3, to 'C:\Media\Surround Test.ac3':
Stream #0.0: Audio: ac3, 48000 Hz, 5:1, 384 kb/s
Stream mapping:
Stream #0.0 -> #0.0
Unsupported codec (id=0) for input stream #0.0

This is what I get when I try changing the channel option on the command line string to 6. I am using an interleaved windows PCM 6-channel wave in 16-bit 48kHz format. It was generated with Adobe Audition's (formerly CoolEdit) multichannel encoder.

Aargh!