A discrete is a single command that always does one thing. A single button that turns the TV on and off is called a toggle. What happens when you press it depends on what state the device is in. If it's on it turns it off and vice versa. A discrete on button only turns the unit on if it's off and does nothing otherwise. Same for discrete off.

In this case you don't need a discrete - just a set way to turn the speakers on or off which could be one command or a series of commands (like menu->down->down->right->enter). Then you put that sequence at the beginning and end of the activity.