You mean in the mixer?
Because in the Instrument window you don't select a sound, only an instrument. One can have multiple soundfonts loaded that all may contain a sound for the chosen instrument. Chances are even higher that the more specialized a soundfont is for a given instrument, the less likely it is to follow the GeneralMIDI standard; meaning that MuseScore has no way of knowing that that soundbank in that soundfont should be associated with said chosen instrument.
[EDIT]
And if you do mean in the mixer; try this:
Loop playback a piece of your score, now change the instrument sound in the mixer.
Comments
You mean in the mixer?
Because in the Instrument window you don't select a sound, only an instrument. One can have multiple soundfonts loaded that all may contain a sound for the chosen instrument. Chances are even higher that the more specialized a soundfont is for a given instrument, the less likely it is to follow the GeneralMIDI standard; meaning that MuseScore has no way of knowing that that soundbank in that soundfont should be associated with said chosen instrument.
[EDIT]
And if you do mean in the mixer; try this:
Loop playback a piece of your score, now change the instrument sound in the mixer.