AIFF files can not be “muxed” with m2v files, because AIFF are not MPG. You can mux audio MPG1 Layer II, beacause the two of them are MPG. When you mux, you get an only file composed with video and audio. When you use AIFF, you just pack video and audio togueter.
As Matt says you can get it in Compressor. Select a MPG2 preset, and look in the “Encoder” for the “Extras” tab. You will find “Multiplexed MPG-1 Layer 2 Audio.
That’s the one your looking for.
Cheers,
Rafael