The problem for me is that I had two videos with different types of audio streams. ffmpeg would join them but they would not play past the point of the join.
So I used mencoder like below and it joined the audio and made them both mp3 streams and it worked!
-oac mp3lame specifies the audio to be convered into an mp3 stream using the lame codec.
after the oac the two files are the ones to be joined.
the -o is the name of the output file
mencoder -ovc copy -oac mp3lame CME-2-router-dial-peer-part1.mp4 CME-2-router-dial-peer-part2-edited-audio.mp4 -o CME-dial-peer.mp4