I don't know of any external HDMI DAC's , You'll have to search for one of those. If you decide to go with a External DAC , then I suggest using Optical out. Use a high quality Optical cable and start reading about External DAC's.

 I have used the Cambridge Audio Dac Magic with many Apple Tv's and the results are stellar. I'm in the process of trying out a Peachtree Idac and maybe a Audioengine D1. 

 I personally own both the Apple Tv 160g and the Apple Tv 3 1080p model. I feel the Apple Tv 160 sounds best. I have them both connected VIA HDMI and Optical. Reason for this is I wanted to hear if there was any audio truth to the HDMI jitter thing I learned about. But my findings is I can't hear any audio differences between Optical out and HDMI out. I sat for hours going back and forth on the same device. Then switched to the other and did the same thing. Same song over and over. The only thing I didn't like about my test was when I switched from Optical to HDMI , it took a few seconds for the audio to come back. From HDMI to Optical was very fast and a better rout for compare.