Here is a radio interview with Rabbi Kaplan (54 min.) mainly concerning kabbalah and meditation.
Recorded on June 6, 1980 - it is transcribed here.
The image presented on the website is not Rabbi Kaplan, rather his colleague.
It is to be noted, that in one of the (posthumous) books, in the preface, there is mention of an extant tape collection of certain lectures. As well, during his life he did quite a number of radio and television interviews in the New York area (significantly more radio, though). Where there are archives and records of radio etc. and of New York etc. there surely exists more content of him. But until then, see above.
This is the only video I've found of him.
Here is a link to a recording.