Can I use avidemux to input a file converted to m4v in handbrake with ac3 5.1 to the same but with aac 5.1?
Kryspy
Not if you want to keep the m4v container.
Could copy video and transcode ac3 audio to aac audio and mux the streams into an MP4 container.
MP4 is pretty much the same as M4V without DRM.