The cast function you refer to is the one nativly built into chrome. It allows you to cast anything on screen, including audio, but it has nothing to do with Madsonic.
I think it is sad that Madsonic advertise chromecast compatibility like they do when it does not work at all.
Even worse is it that nobody seems to care at all what is posted in this forum.
Madsonic is a fine program, but the backup and support from its makers are definitely way below par.