How to use an old Mac as an AirPlay device
If you would like to set up an AirPlay device for playing music, you can do so without purchasing any new hardware.
Apple's AirPlay feature is a convenient technology for sending audio to various devices. If you are on the same wireless network as the device then it will be available to your Mac, iPhone, or iPad to play music via iTunes, or even be used as a general audio appliance for system sounds.
In general, AirPlay devices are hardware accessories like routers such as Apple's AirPort, or media appliances like the AppleTV. However, if you are not willing to shell out about $100 or more for a new AirPlay and you have an old Mac lying around (especially a Mac Mini), then one option is to turn it into an AirPlay hub.
To do this, simply connect the Mac to an audio system or a set of external speakers. While you might attach the Mac to a television to have some sort of display for it, this is optional if you enable Screen Sharing on the Mac. By doing so in the Sharing system preferences, you can access it through another Mac or with a VNC client on a Windows or Linux machine, and be able to configure it over the network.
Another detail you might consider is to enable automatic log-in, which can be enabled in the Users & Groups system preferences in the "Log-in Options" section at the bottom of the accounts list. This option will allow the system to log in and launch the software needed to turn the system into an AirPlay device.
With your Mac set up, next install the AirFoil tool from RogueAmoeba ($25 purchase), which will turn it into an AirPlay receiver. Install the software by dragging it to your Applications folder from the installation disk image, then be sure to add it to the Log-in Items for the account that is set to automatically log in. This can be done by selecting the active account in the Users & Groups system preferences, then dragging the program to the list in the Log-in Item tab.
With this set up, you can now keep the Mac running as an available AirPlay device that you can play music through to your heart's content.
If you do not have an old Mac but instead have a Windows machine or even an old iPhone, iPad, or iPod Touch, then you can set up a similar option with these devices, as RogueAmoeba has software versions of Airfoil for these platforms as well, though Apple required it to remove the use of AirPlay from the iOS version of the software. While currently there are no alternatives for iOS devices, RogueAmoeba does have a workaround using its software.