@bambi2000 Thanks. I did manage to get the basics working (as well as find the streams for P3 and P4 :-) ). I also managed to group players by setting CurrentURI of the "slave" IP to the UUID of the "master".
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/" s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"><s:Body><u:SetAVTransportURI xmlns:u="urn:schemas-upnp-org:service:AVTransport:1"><InstanceID>0</InstanceID><CurrentURI>x-rincon:RINCON_XXXXXXXXXXXX1400</CurrentURI><CurrentURIMetaData /></u:SetAVTransportURI></s:Body></s:Envelope>
Now I just wish Flic would create some sort of copy function to make it easier to reuse those commands:-) And that Sonos would open their official API for better integration and remote control.