Hi,
My setup will be a: LG HDTV (who can’t passthrough 5.1 and convert it to Stereo), a New Apple TV, Logitech Z5500 (home theater sound system that can only decode Dolby Digital and DTS).
To make it work I bought this to get the sound out of the HDMI and send it through Toslink: http://www.amazon.com/gp/product/B00KBHX072/ref=ask_ql_qh_dp_hza t
My Question: Will I be able to get 5.1 on my Logitech Z5500? And if yes what is the best settings to set in Infuse and on my futur Apple TV?
I can’t buy a new sound system nor a A/V receiver it too expensive for now 
Thanks a lot for your help!
Rather difficult to say if it’ll work, depends on too many factors, e.g. on how reliable that “HDMI Audio Extractor” works. Either way, in your specific case, it makes most sense to set Infuse to ‘passthrough’, as Toslink does not provide enough bandwidth for uncompressed multichannel audio anyway. DTS and AC3 should bitstream just fine, provided that converter works correctly.
You should be fine playing DTS-HD (MA) files that way even, as the lossy DTS core will automatically be passed through. Dolby TrueHD is a different beast, because the AC3 core is not mandatory, most BluRays have an additional AC3 audio stream included for compatibility reasons.
Thank you for you very complet answer!
I tried my HDMI Audio Extractor with my MacBook Pro and VLC instead of an Apple TV because for now I don’t have an AppleTV. It seems to work pretty well! 
I’ll keep update this post when I’ll get my Apple TV if it can help other people with a Z5500. I herad it’s a popular old home theater sound system.