Can I keep mono audio on AAC?

[synnchan]

I have a few .avi's here with PCM 1 channel (constant bitrate) for audio, and when I remux them into .mp4 with avidemux, they become AAC 2 channels (variable bitrate).

How can I turn them into AAC but keep only 1 channel (constant bitrate) like they originally were?

[hello_hello]

I don't think AviDemux has an option for constant birate AAC, which is fine because for the same average bitrate, variable bitrate should be higher quality. It's the same principle as using variable bitrate for video. You don't want to use the same bitrate for encoding frames of black or static scenes as you would to encode a car chase.

Unless you have the mixer enabled in AviDemux's audio filters the output should be mono. If MediaInfo (how are you determining it's stereo?) is showing something like:

Channel(s): 2 channels
Channel(s)_Original: 1 channel
Channel positions: Front: C

Then it's mono.
Mind you even if it's being upmixed to stereo it's probably no big deal. That'd give you "dual mono" which requires very little extra bitrate for the same quality. For "dual mono" the audio would always be in the left and right stereo speakers. I think when it's mono and you play it on a surround sound system it can end up in the centre speaker only.