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 Crickets.m4r Old\ Car\ Horn.m4r Sonar.m4r
Ascending.m4r Digital.m4r Old\ Phone.m4r Strum.m4r
Bark.m4r Doorbell.m4r Piano\ Riff.m4r Timba.m4r
Bell\ Tower.m4r Duck.m4r Pinball.m4r Time\ Passing.m4r
Blues.m4r Harp.m4r Robot.m4r Trill.m4r
Boing.m4r Motorcycle.m4r Sci-Fi.m4r Xylophone.m4r
I called mine "Silent.m4r" and it shows up under your ringtones, and I then set it under specific numbers I get constant calls from so it does not disturb me anymore. This is yet another reason why it's great to jailbreak your iPhone since Apple doesn't have any Blackberry like settings to control different volume sources at all.
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