I assume you just want to run the audio through the receiver, while sending the video direct to the TV. If so, there are devices that split the audio from an HDMI connection into optical or stereo pair (haven't said that in a while). Just be sure it handles HDCP correctly, or ATV will refuse to send it any video. Search around and shop carefully; caveat emptor.
Alternatively, does your TV have a suitable audio output you can use? How do you get TV sound to the receiver now? My Samsung TV has an optical output that I can feed to my receiver, and that works fine for ATV sound. And amazingly, it relays DD 5.1 from ATV.
I see your Bose 321 has only composite or S video inputs (those were the days). Do not even consider attempting to convert the ATV video down to these formats. Such converters will not handle HDCP and you will not be able to play any copy-protected content. Just use the Bose as your sound system.