Thank you!
The detection_radius is actually already a rectangle (well, a square) - the "radius" is just half the width. This variable is found in the par_speaker variable. By default the speaker objects will inherit this value, but you can tweak it for individual objects.
Since as you said you might want to make it a rectangle instead of a square, you might want to tweak the step event for some objects too. Instead of just adding the radius to all the rectangle variables, you could add half the collision box height, and half the width, for the x and y values of the rectangle.
I hope that makes sense - let me know if you have trouble!