Instead of using only some methods of yours I used the whole class, so that I heard the sound continuously :) Great! ^^
Now I am just wondering why the physical limit of it is around 2660 Hz? Not sure of which of its properties limiting it at that certain value. Is it because of its digital resolution?
I tried to double "UNIT" and half the frequency, and I could raise the higher limit a tiny bit, but I couldn't succeed in the end.. I could see that in your example the top frequency is 1000Hz. Was it because of the same reason?