This is a simple fix. I have the same plane & the first thing I did was to replace the panel & sound folders.
All you have to do is to copy the panel & sound folders of the boeing 777 and paste them into the Boeing 7E7 folder and over write.

This kind of problem is common with downloaded airplanes.
A designer makes a great airplane than sticks a default sound a panel file into it. Does not make much sense to me.
It like cooking a great meal only to put garbage on it afterwards. Kind of dumb if you ask me.