if you want to go the ic route just use a 555, its fairly common and theres at least a few in every electronic device you can get. i think you can change its timing interval and you could run its output pulse directly to a speaker. then your keypad would probibly be used on the control side and somhow use it to tell the 555 at what speed to operate. ive only used one once in a robotics kit but i never quite understood how it worked. i even downloaded a proggie called electronics workbench, which supposidly can be used to design and simulate the function of circuits, i think you can even print a negative if you want to cut a pcb, anyway i tried to make a circuit that does what youre looking for, and failed miserably.. as usual wikipedia is a little bit sketchy about it too, anyone know a good online resource for wannabe electronics engineers?