Android - 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).
In stock Android it's not possible to have more than two audio channels (stereo) via USB as documented here.
The 3.5mm jack as well as Bluetooth do not support more than 2 channels either (although enhanced aptX (bluetooth) does support 5.1 surround sound, it's not yet supported by android).
[edit] You can connect an external sound card, it may even work but you won't have 5.1 output through your Android's USB port which means you could only have virtual surround sound on an external sound card.