channelmap

channelmap is a simple UNIX–y program to assist in the extraction and merging of audio file channels between audio files.

Its usage is simple:

# join two audio files into one
channelmap out.wav left.wav:1 right.wav:1

# split the file back out
channelmap left.wav out.wav:1
channelmap right.wav out.wav:1

Source can be retrieved at its github repository.