I have a Apple TV and a Bose 321 system....Can't get sound

I have had a Apple TV for about a month now...

Originally I just fed it via HDML to my TV and just played the sound off of that, but my 2007 Sharp Aquos TV doesn't have great sound compared to my BOSE 321 system. The Bose, however, does not have a HDMI port so looking at my ATV I saw there was an optical port. I connected the ATV to my BOSE via the opitcal port and assumed I would be selecting "AUX" as my input. I did this and there was no sound. Any help would be great. Thanks!