I found a script for cacti to monitor the number of current listeners of an icecast2 server. The script was ok, but had a problem that sources where counted as listeners and it only showed the total number of listeners.
I did some work on the script and it now has:
- show all listeners when entering “all” into the filter field
- filter for mountpoints or parts of the mountpoint name
- the script now sums the listeners of each mountpoint in the filter – positive sideeffect: if you filter for “all”, all mountpoints get summed resulting in the correct number of listeners for all your mountpoints 
download: cacti_icecast2_v2.zip