You go to the Apps store and download the Remote App onto your iphone. Its FREE from Apple.
Remote lets you use your iPhone as a remote control for iTunes on your iMac.
All the songs on my iPhone came from my macPro ... so I just need to play them in iTunes to feed the house.
When sitting at your iMac, you go into iTunes and set up the speakers in your house as "speakers".
You will need to have either Airport Express or AppleTV units at those locations.
I have my house set up exactly like this -
I have an Airport Express upstairs with my tubes and JBLs,
I have another downstairs in the TV room, and a 3rd one on our screened in porch out back with a boombox.
So I can use my iPhone to select and play songs (or playlists) from my MacPro, and they will go over wifi to the 3 Airport Express units in the house. I just need to turn the hifi in the room I am in to AUX (or whatever source the Airport is plugged into) and the music will come out there.
I can even let it play in all 3 places at one if I want to ...
In your house, you put an Airport Express into your console, run a minijack to stereo RCA-phono adapter-cord into the AUX or whatever on your console, and you are done.
If you want to play directly from your phone, unplug the wire from the Airport express and plug it into the headphone jack of your phone ...