I can hear/imagine a human voice singing, (similar to the way my internal voice 'talks' when thinking) but i usually can't imagine instruments, because they always end up sounding as if they are performed by a human voice.
Several times i did listen the sound of instruments as if real, but it happened only before falling into sleep, and just thinking about that would wake me up enough to bring the human voice back.
Do you / have you ever played any instruments before? Maybe it's just case of associating something else with the sound the instrument makes - for example perhaps you need direct experience of producing say, a guitar sound, in order to associate the 'performance' of the act, with the actual sound it produces.
