Infrared LEDs can be had for as little as 20 cent each (might find cheaper).
**broken link removed** I haven't purchased any of these products, but have bought other things from the site, been around for a while. Black and white cameras are the most sensitive to IR, and a better choice if you a primarily just interested in seeing what's out there. You can go color, and remove the IR filter, usually glued to the lens or sensor (90% alcohol solvent works), but it'll alter the colors a little, and night will still be basically black and white. The IR LEDs come in quite a variety. 940 nm is completely invisible, but cameras aren't as sensitive. 880 nm is better, but you can see a slight red glow if you look straight at them. Haven't tried 850 nm, but might appear brighter. The smaller the angle, they'll appear brighter, but light a smaller area, might be fine depending on your camera lens. The wider angles appear dimmer, but cover more area. They aren't too bad to build yourself, just a lot of soldering. I use all holes perfboard, mostly because the soldering is tedious enough, drilling all those holes... **broken link removed** Will help you construct your light, and choose resistors, pretty useful. If you aren't in a hurry, Goldmine has these on sale every few months for $5,
Quorum A-160 Passive Infrared Detector-The Electronic Goldmine , gotten several, and pretty easy to hook up. Wished I could figure out the wireless portion, as it's a daughter board, that can be removed, without effecting the PIR function. Pulling it all together would be simple with a microcontroller, but you could use timers as well.
Loud audio isn't really my thing, but cheap speakers, sound like it...