Help me find a good audio encoder.

[epow0317]

Hello~,

I have a problem in encoding a three or five-channal audio file into stereo(2 channal).
Nero AAC GUI i have in my computer seems to can't control audio file's channal.
Only Encoder (LC-AAC, HE-AAC and so on) or Bit rate it could control so that having been encoded and muxed with video, it didn't work at portable devices.

So, i'm finding a good audio encoder better than Nero AAC GUI which is equipped with channal-controlling function.
Could you introduce me a good one?
Anway, thanks in advance~!

P.S) if there's a error in grammer or crude use in vocabulary, please understand me. - i'm not good at english.