Your garage door should already have a IR sensor that keeps it from closing when an object is in the doorway and it turns on the garage door lights if you or some one enters or exits when the garage door lights timed out.
Just rig a 120 VAC buzzer or horn to the opener light circuit, and a switch for when you don't want it sounding.
Open the garage door wait for the Light to time out, most can be set for the time period, then turn on the switch for the alarm. When the sensor beam is broken, the light will come on and the alarm will sound.