I too am attempting to extract the payload from an RTP stream in a packet capture. I have Googled it I assure you.
There are commercial methods, no good for me.
There is Cain and Abel which can do real time recording off the wire, but I don't want to run windows, and I want to work with packet capture not real time. I also don't want that program on my computer.
rtpdump also seems to only work off the wire and not from packet capture files.
Wireshark do exactly what I want, but as far as I know this cannot be done by the command line with wireshark. What a bother that this program does just what I need but has not command line for that feature.
Does anyone know of a simple piece of free command line software that can take a pack capture file and extract each rtp payload it finds?