Using hdajacksensetest I found out the mapping: SPDIF port (jack 1): Pin 0x16 (Green Line Out, Rear side) jack 2: Pin 0x18 (Green Line Out, Rear side) jack 3: Pin 0x15 (Green Line Out, Rear side)
Do both headphone jacks have headphone , headset or line out icon ?
You can see the icons here [0] at page 20
Are you sure that those icons and jacks order matched with your notebook ?
The icons match the laptop, I can send you a picture if you wish, but it looks identical.
The user guide seem not specific to g751jm
- the icon is a line out symbol, arrow point out of the circles but the
description is audio input jack (i.e. line in) which does not match with the specification
Neither specification nor user manual mention the support of surround 5.1
It is curious because I have selected Analog Surround 5.1 Output in Sound Settings (GNOME), and I can test each individual speaker (5.1). It also works in Windows 8.1