http://www.avsforum.com/forum/166-lcd-flat-panel-displays/1530461-official-2014-sony-xbr-49x850b-xbr-55x850b-xbr-65x850b-xbr-70x850b-thread.html lead me to the product's PDF.

https://esupport.sony.com/US/p/model-home.pl?mdl=XBR55X850B&template_id=1&region_id=1&tab=manuals#/manualsTab has more detail.

Page 35 of https://esupport.sony.com/US/p/model-home.pl?mdl=XBR55X850B&template_id=1&region_id=1&tab=manuals#/manualsTab notes the headphone minijack. I'd use that.

It also mentions fixed or variable audio levels. Set that as you wish.
Bob