#if you have nvidia make sure you install the nvidia-cuda-toolkit so hardware acceleration can be used
wget http://ffmpeg.org/releases/ffmpeg-3.3.2.tar.bz2
tar -jxvf ffmpeg-3.3.2.tar.bz2
cd ffmpeg-3.3.2/
./configure --disable-yasm
install prefix /usr/local
source path ........
As shown below you can find ringtones in the "/private/var/stash/Ringones*" directory of your iPhone.
Different reports have said that the .m4r format is really an MP4 file and some have said AAC and some also say "it's the same as the M4A" format. Whatever the case is, one easy way is to install the silent theme from Cydia and copy that into your ringtones directory.
/private/var/stash/Ringtones.sh7cLj root# ls
Alarm.m4r ........