Ubuntu – Trying to convert MKV to MP4 with avconv, failing

As I've already written here, I suggest you change a few details in your command line:

• If you're using an Ubuntu release older than 15.04, replace ffmpeg for avconv, as it was the fork used by Ubuntu for a few years (see the comments)
• Do you really need -absf aac_adtstoasc? It had some issues that may not have been corrected in current Ubuntu's ffmpeg. I suggest you to try without it first.
• -vcodec copy -acodec copy can be simplified to -codec copy or -c copy. It would also prevent tampering other streams besides video and audio, like subtitles.

So the winner is...

ffmpeg -i input.mkv -c copy output.m4v

If that does not work, please post all terminal output.

To sync the audio stream with video time stamps we may use the -async option:

-async samples_per_second
Audio sync method. "Stretches/squeezes" the audio stream to match the timestamps, the parameter is the maximum samples per second by which the audio is changed. -async 1 is a special case where only the start of the audio stream is corrected without any later correction. This option has been deprecated. Use the asyncts audio filter instead.
^{Libav documentation}

Begin to try with samples per second matching your framerate and decrease until your audio is in sync:

avconv -async 30 -i <infile> <encoding options> -r 30 <outfile>