Today I got a notice that the 3G movies in the Passie voor Horeca app no longer worked. When on a 3G connection, the movie would not play and exit immediately back to the list of movies.

It turns out that the AMR audio format (libamr) that ffmpeg uses by default for all .3gp based movies, is no longer supported. In order to fix this, you need to re-encode movies with a supported audio encoder like AAC, this is done by adding the -acodec libfaac option to the ffmpeg commandline.

Example commandline to get a working .3gp file for use over 3G and/or GPRS:
ffmpeg -i inputfile.flv -vcodec mpeg4 -acodec libfaac -r 10 -ac 1 -ar 8000 -b 52050 -ab 11950 -aspect 16:9 outputfile.3gp

I had to downscale the video bitrate a bit (-b option, decreased it with 4000) and up the audio bitrate (-ab option, added 4000) to get a decent audio track in the resulting .3gp file compared to the AMR format.