This is an attempt at making a better Sonos controller for iOS. It's pretty hacked together right now -- I'll try to articulate needs later so others can contribute. The following (sparse) instructions below should get it working with your system.
First figure out the IP addresses of all your speakers.
Easiest way to do this is to checkout: https://github.com/rahims/SoCo and use the command line example. Run:
python sonoshell.py all list_ips to find all the IPs. Then run
python sonoshell.py YOUR_SPEAKERS_IP_ADDRESS info to determine the uid for each speaker.
Now open the file PLNowPlayingViewController.m and replace lines 79-82 with your speaker information. Compile and you should be able to control the volume of your speakers.