Yes, this is a school assignment, I'm just trying to find out what the best software for having the computer beep when it reaches a certain point since this is a forum that should help me.
There isn't any off-the-shelf, ready-to-run software that is going to read free-form data from the serial port and activate the speaker.
You need to define the format of the serial data, and use an application like Processing to create a sketch that listens to the serial monitor, reads data that arrives, and makes happy noises (or sad noises).
Or, as Ta_anders says, use a piezo speaker and forget about the PC.