Yes, you can stream audio to a second or third generation Apple TV (the smaller black ones). The first generation ones (larger silver/white plastic box) doesn't support the necessary AirPlay protocol.
On iOS, the app playing the audio needs to be programmed with AirPlay support (the iOS Music app and Safari both support it), but if it is, you should see an AirPlay icon (). Tap it, and you should see a list of possible outputs, including your Apple TV. If you don't see the AirPlay icon, and your Apple TV is on, make sure they're both on the same local network.
On a Mac, iTunes can output to your Apple TV. You should see the AirPlay icon in the lower right corner. If you want to play audio from other apps, you'll need a third party program like Airfoil, which allows you to send any audio your Mac plays to the Apple TV (or any other AirPlay device).
If you have more questions, read Apple's support article, Using Airplay (HT4437).