avi with ac3 question?

[jmusd]

I am trying to passthrough the ac3 track when I am encoding a movie using "mpeg4 mencoder." It does not seem like it it is keeping it in 5.1 even if I have that chosen.

Opening audio decoder: [liba52] AC3 decoding with liba52
Using AltiVec optimized IMDCT transform
AC3: 5.1 (3f+2r+lfe) 48000 Hz 448.0 kbit/s
No accelerated resampler found
AUDIO: 48000 Hz, 2 ch, 16 bit (0x20), ratio: 56000->192000 (448.0 kbit)
Selected audio codec: [a52] afm:liba52 (AC3-liba52)

It seems like it is making the audio track stereo. When I look at the movie info using mplayer osx2 it says 2 channel. Am I doing something wrong? It does this no matter if I have 5.1/stereo selected and/or passthrough/ac3 selected.

[jmusd]

anyone ... major?

[broaddd]

I have been successful where 'encode audio' is ticked, and the option set to 'passthrough', and the various options below that matching the source (eg. 384kbps, 5.1 etc - but I guess this part isn't necessary).