Even if it is set as default or not, and when testing, although Windows shows that it outputs to it, nothing is heard.
Unless I am mistaken, the port should have a red light (it is optical, so how would it function if it doesn't light up?).
On a G74SX, as is the case with the G75VX, the headphone jack doubles as the spdif output, with optical connection. On the G74SX, it is always lit red.