I am wondering about the disparity in skill between audiation--purely internalized hearing--and sight singing--the vocalization of a melody. I think these two phenomena are often assumed to be interdependent kind of like flip sides of the same coin. I however have found my experience of these two skills to be distinct and unrelated. My sight-singing is more developed than my audiation, and furthermore I do not necessarily hear notes internally before producing them vocally.
Logically, one would expect that in order to sight-sing correctly on pitch, that you would have to pre-hear notes before singing; I however often do not pre-hear tones but can nevertheless sight-sing basic melodies. My sight-singing is also consistently correct so long as the melodies are within my range of skill. This seems to prove that one does not need to hear notes before producing them and that my sight-singing is not correct merely by chance. If this were a case of chance then I would often be sight-singing melodies incorrectly. One could easily imagine the opposite problem, that is, hearing notes internally but not producing them correctly, and that would be due to undeveloped vocal technique.
My gradual improvement in sight-singing have not resulted in much improvement in my internal hearing. The two skills are not entirely independent, so for example I can sometimes pre-hear notes before vocalizing but often only one note ahead. I'd like to be able to hear multiple notes ahead. Can anyone shed light on this phenomenon as well as offer methods for developing internal hearing (possibly without suggesting more sight-singing)?