Dirt cheap:
Use one of those cheeezy greeting cards that lets you record a "special message". Rip the guts, wire the headphone out on your computer to the mic in on the card guts, and record the Pac-Man intro.
Even cheaper: Steal one of your kids' toys that records speech and use that.
Hook the speaker output from whatever you use to whatever horn/amplification you can find- real musical horn, megaphone, boombox-duct-taped-to-the-grill-of-the-Roadmaster, whatever you can find.