I'm trying to replace my Sonos system with old iphones. I can stream Pandora with the iphone app but also want to be able to control the music remotely from my Mac. Is there a way of doing that?
You can get a VNC app that will allow you to remote control your mac. Something like "Screens" https://itunes.apple.com/us/app/screens-vnc-client-for-ios/id400012962?mt=8
That will allow you to control your mac entirely from your iDevice.
It's possible to use an iPhone as a VNC server... provided it's jailbroken. There's a post here that talks about it and goes through installing and setting up one such app, named Veency.
Remoteless for Pandora is a remote iPhone / iPad app that lets you remote control Pandora running in Safari, Chrome or Firefox.