Princess Auto
At Princess Auto look in the surplus section for a small set of amplified speakers for an iPod or MP3 player. Find one that takes a couple of AA batteries or a 9 V. It will have a eighth inch stereo input Jack and then two wires connecting to a small speaker or two. That is your mini amplifier, just remove it from whatever it's encased in.
Pick up a cheap MP3 player, Staples has a two gig MP3 player with a USB connector for transferring the files for around 10 bucks. Then pick up a small house alarm horn at either Princess auto or an electrical store. Some car alarm horns will work as well. One last trip to the dollar store for a 1/8 stereo male to male cable and you should have all the parts you need. Make sure the horn you get will fit inside a piece of PVC pipe, cement it in and get an end cap for the other end. Take the two wires from the mini amp that were hooked up to the small speaker and hook them up to the Horn. Use the cable from the dollar store to connect MP3 player to the mini amplifier. When you copy your calls to the MP3 player make sure you put a couple or few minutes of silence at the start. You should then be able to press play on your MP3 player and have a few minutes to get to your blind before it starts calling them in.