I cannot reproduce the problem you see.
I made all sort of experiments now during descent.
The MESSAGE+AURAL BEEP came up each time when they should, that is when Speedbrake extended and thrust not closed.

One thing must be said:
On the REAL 777 during normal VNAV descent Thrust is IDLE (where "Idle" is not really idle but a value computed by the FMC which varies with altitude  (N1=24-38))
So, when you set the speedbrake , the message does not come up immediatly, as thrust is idle, but only after speed reduction, when the Ap must apply more thrust to hold the commanded speed.
The Cs777 is not "realistic" here as during normal VNAV descent Thrust is NOT IDLE but about N1=50-70 (at a ROD of 1800 ft/min).
So when you set the speedbrake the message comes up immediatly as there is ALWAYS THRUST applied.

To test if with the CS777 the message comes up only if the 2 conditions are fulfilled(Speedbrake set and Thrust applied) I made a (very) late descent, starting the descent from 5000 feet above normal profile. In this case the CS777 reduces thrust to idle, and the message came well up only when more thrust was applied.

I use Fsx-Sp2 (without Accel)

That's all ! :)