Little more background info:
In the past the instrument had a prehear option in the instrument list. It could be turned on with a mouseclick and with a shortcut key.
The option has been removed and just for a little while the shortcut key still worked regardless of the missing icon.
But anyway, prehear for the instrument section is not necessary, like Mat-weasel pointed out:just press any of your keys on your keyboard in the pattern editor and you will just hear the audio play.