Perhaps the developers from ISub and AVsub can have a look at the DS Audio app from Synology. There you can simply select your Sonos device to be used as output device. This would be a great feature for both native subsonic apps. No clue how difficult it is but apparently it can be done. Who will put it on their ToDo list
just wanted tp get attention for this a bit more. I do believe that it also could be integrated into the regular player options within the Subsonic gui. In case a Sonos system is available in the local network, it should be an option to select that source directly.