This code (to extract a section) has worked well for a long time

ffmpeg -vcodec copy -ss 71 -t 624 /home/bill/Music/recordings/source.m4a /home/bill/Segment-01.m4a

But has now stopped working.

First I get "...source.m4a already exists - OK to delete"
Then with:-

ffmpeg -i aa.m4a -ss 00:00:00 -t 00:05:00 -acodec copy new.m4a

I get:-

ffmpeg version 0.8.3-4:0.8.3-0ubuntu0.12.04.1, Copyright (c) 2000-2012 the Libav developers
built on Jun 12 2012 16:37:58 with gcc 4.6.3
This program is only provided for compatibility and will be removed in a future release. Please use avconv instead.
aa.m4a: Invalid data found when processing input

Any help would be appreciated.

Ubuntu 12.04
KDE SC Version 4.9.2