A new piece of software called ham2mon by programmer Louis Brown (KD4HSO) has been released. This is a GNU Radio Linux based program that can be used with an RTL-SDR or other SDR that is compatible with GrOsmoSDR to demodulate and record narrow band FM audio from N channels (as many channels as your CPU can handle) within the current bandwidth. Although it is command line based, it comes with a simple Curses command line based GUI.
He writes that in future versions he hopes to incorporate gr-dsd to be able to demodulate and record P25 digital audio as well.
Louis has also uploaded a video to YouTube showing the software in action.