How to play 5.1 audio direct to speakers
There are many ways you can get the audio out.

Through wire:


Head phone jack - you clearly said it can deliver only two channel audio.
USB - You can use an OTG adapter and plug in a 5 channel USB sound card and get the 5 channel audio out.  
USB Type C may have this support native.


Wireless:


Bluetooth - if the A2DP profile is supported (will be supported from v2.0 onwards) you can stream audio in stereo - again not good enough for you.
Screencast protocols like Miracast or DLNA might support as it can be considered as a HDMI connection over Wi-Fi. These should support 5 channel audio.


Of course for all these you have to have a player which has appropriate codec support (usually AC3).