Converting mkv to mp4 using ffmpeg

There are a thousand examples online that help you convert a mkv into an mp4 (and explain how mkv is just a container for mp4). Unfortunately, those examples either didn't work (error below) or took absolutely forever to run (because they re-encoded the video). Knowing that video stream was exactly wanted I wanted and I just wanted to fix the audio, this worked for me (it tells ffmpeg to copy the video stream, but re-encode the audio to something both Firefox and Chrome are happy with):

$ ffmpeg -i input.mkv -vcodec copy -acodec libvo_aacenc -ac 2 -ab 160k output.mp4

Without the specific options to convert the audio, you get this:

Error while opening encoder for output stream #0:1 - maybe incorrect parameters such as bit_rate, rate, width or height

As with anything ffmpeg, your mileage will likely vary, but if this helps even one other person, I'll call it a great success.