A company called Rogue Amoeba has several programs that work with audio. I have a few of them, but I started with a program called Airfoil. While iTunes is made to work with Airport Express and Apple TV (2), Airfoil works with any program (that I have tried) to steal the audio signal and send it to Airplay products on the same network.
Another cool feature is Airfoil Speakers. There is a version for the Mac and for iOS devices. It allows you to send the audio signal from the MBP to an iOS device. The Mac version will show the Mac as an Airplay device, so you can send the signal from an iPhone to a Mac. This would come in handy if you have a Mac Mini or MBP in your home theater setup instead of an AppleTV or Airport Express.
Rogue Amoeba also has a cool program for using SiriusXM. It keeps you logged in so you can just open the program on your Mac and start playing music.
They also have a "freebee" called Soundsource. You can launch it and it will list and let you choose your Mac's audio outputs. I use it quite a bit with my Mac Mini since it feeds two systems. I use HDMI out for audio in the living room and optical output in the office. With Soundsource, it makes it very easy to select where you want to send the audio signal.
Finally, I believe most of their stuff is for Macs, but Airfoil works with Windows, as well.