Controlling MacMini volume with MacBookPro volume keys?
You could try using the Airplay for that.

The models that support Airplay

– If you want to mirror to a Mac: OS X Mountain Lion, a supported Mac, and an app to support mirroring.. Supported Macs are:

iMac (Mid 2011 or newer)
Mac mini (Mid 2011 or newer)
MacBook Air (Mid 2011 or newer)
MacBook Pro (Early 2011 or newer)