Would it be possible to hook up a 3.5mm jack to the arduino uno? So far I think I will hook up a SD card reader, and have the board read that data when a sensor is tripped. But I don't think that the default speakers will be loud enough.
I am just beginning my coding, so I have nearly no idea. Thanks
Look up the AdaFruit wave shield. There is a tutorial and example projects that do exactly what you want.
I used the shield Mike is suggesting in this Sheep doorbell project a while back.
Great project 
+1
Thanks Mike!