If you have an analog stereo amp or reciever, one that uses RCA plugs, using an Apple TV 2 requires that the HDMI or Toslink optical be processed by a DAC or digital audio converter.
If you are an audiophile, you will know all about this but, if you just want to get the job done and have decent stereo from Apple TV 2, here is my experience.
If you have a TV with an RCA audio jack out - one white and one red - you can plug the Apple TV 2 HDMI cable into the TV and run an RCA cable to the reciever in jacks for CD, LD, VCR, Tape. (Just not LP!) The TV DAC will do the audio via the digital HDMI and you are done! Make sure you set Apple TV to two channel stereo or it will be just the noise of unprocessed Dolby Digital gunk.
I have a bottom of the line Sony Bravia and its DAC is fine for Netflix, OTA HDTV (excellent audio with Dobly Digital turned off) and most uses.
If you want to get a bit more better sound, plug a DAC into the Apple TV via the Toslink optical out. Then run RCA to the reciever (same as above). If you spend hundreds of dollars on a DAC, it will sound great. But if you have done this, you are an audiophile and know this already. Why are you reading this?!
If you want an great but cheap DAC, pick up a FiiO D3 Digital to Analog Audio Converter - 192kHz/24bit Optical and Coaxial DAC for $30 (Amazon sells them).
For most music, like MP3s, you won't hear much difference (okay, none at all) than if you run audio through HMDI to the TV DAC then out to RCA. But for good recordings (like ones I burned from CDs I own), the FiiO D3 does add a little more zip and range resulting in a livlier performance.
I run the FiiO DAC to an perfectly fine Yamaha RX-595 receiver with direct amplification circuit (skips the base, treble, balance) and in turn to some Magenpan MMG speakers. Jazz and classical via iTunes to Apple TV 2 sound great!