In order to enable and disable portions of an audio program easily, and to be able to reuse them multiple times, you will probably want to encapsulate an entire audio-generating or audio-processing procedure inside a single patch with inlets and outlets so that it can be used as a subpatch object in some other patch. This patch shows an example of a simple 2-oscillator frequency modulation tone generator that could easily be used as a subpatch in some other patch.
Using both the key and keyup objects, you can tell when a key was pressed and when it was released. The split object allows you to isolate a range of numbers; it passes only the specified range of numbers out its left outlet, and passes all other numbers out its right outlet. The numbered keys 0 to 9 correspond to ASCII codes 48 to 57, so it's easy to isolate those keys, and subtracting 48 brings those numbers down into the range 0 to 9.