Porthole is a Mac app which will stream all the audio coming from your computer to AirPlay speakers, instead of just the music from iTunes. It’s kind of like Rogue Amoeba’s excellent AirFoil, only much less fine-grained in terms of control.
Once installed, Porthole sits in your menubar and detects any AirPlay devices on your network, like Apple TVs and AirPort Expresses. Pick one, and instead of coming out of the Mac’s speakers, sound is sent to these devices.
The audio will be slightly delayed, as Wi-Fi-based AirPlay is buffered in the receiving device, so forget about watching movies. But for music streaming it’s ideal, and at €8 ($10.50) it’s all a deal. If you want to send the sound from a single app, and not your entire system audio, then check out the aforementioned AirFoil, which will do just this, and even has a movie player which syncs sound and picture.