I once read that an average audience member retains only one sentence
of information one day later from a one hour lecture. I'd like to know
the origin of this. If unavailable, I'd like to know of any studies
that discuss the average retention for standard 60 minute lecture.

While not perhaps providing exactly what you are looking for, these
sites have some information regarding the issue and mention
researchers who have studied this:

http://www.simpson.edu/dal/faculty/lecture.html

http://www.ntlf.com/html/pi/9601/article1.htm

I hope this helps.

Have a good day.