The problem is NOT the apple tv it looks like the set up to the BOSE. The problem from what you say is with the AUX audio optical mode on the BOSE as that is the input that is being the problem. I have apple TV optical audio to a Samsung Sound Bar and it works completely indipendently of the TV and TV sound through the HDMI.
The problem is with the Bose bar and not the Apple TV . I have confirmed that sound is Transmitted from both the HDMI output and the Optical (Toslink) output simultaneously from the Apple TV
The BOSE bar AUTOMATICALLY selects the best quality signal . You can't select it. So HDMI will win and once the TV is off, audio will not work.
The AUX port is essential useless as Bose told me its for "Redundancy" and you can't "select" to use it.
What you need is an optical selector . I bought this one for around $20 from amazon . I like it because its black and you can connect up to 4 devices.
Because the Bose bar inputs from the Optical out on the back of the TV, have it now go through the selector. Your Apple TV can be input 2 .
You can now watch TV or Apple TV normally but if you want to still hear the music from Apple TV , manually change the selector to input 2 on the optical selector device . When you turn off TV it will still play sound coming from apple tv's optical output .
Works perfectly ! Cheap work around . Unfortunately for such an expensive Bose device you shouldn't need to buy "work arounds" but I'm very happy with the sound quality and I'm a Bose believer . So it was worth doing .
I just purchased the optical selector you mentioned. However, I forgot to buy an additional optical cable! I actually forgot that ATV has an optical input since I've always just used the HDMI.
I actually have a Sonos Soundbar. I was bummed that I too couldn't simply play music through my ATV for reasons listed above. I was about to exchange it for a bose cinemate until I realized I'd have teh same problem. Being it's the same problem, I'm sure your solution will work just fine.
I'm looking forward to setting this up.
Optical audio switching is the only way to go.
Here's my set up.
DRV, BLURAY & Apple TV optical audio out to a Toslink 3 to 1 switcher going straight to the main TV optical input on my Bose Cinemate 1SR. Works like a charm.
I opted for the $30 remote control enabled optical switch I bought from eBay but if you have a manual $6-10 optical switch...that would work too. Just don't buy the Y splitter, it would simply not work. I'm also looking at possibly going for an automatic optical audio switch in the near future from:
Audio Authority 1177A-1 TOSLink/Coaxial Digital Switcher/Switch Automatic SPDIF
Bit expensive at around $180ish but at least it'll automate my set up better.