The two tone siren sound is indicating an overheating cpu. The alarm will sound at 75c and turn the pc off at 85c.
I have just built two identical systems for a friend using the AV8 Turd Eye and 939 3000+. One ran sweet as a nut the other has been an enormous pain in the arris. If you pop over to the Abit Forums (
http://forum.abit-usa.com/ ) you will see that a lot of people have been having problems with the AV8 and the temperature sensors which are about as accurate as David Blunkett playing darts.
My solution was to flash the Bios to the latest version which is 1.7 and to replace the heatsink with the stock AMD one (this I did just to eliminate any possible probs with the Thermaltake heatsink I had installed though it was fine on the other box). Even so the Turd Eye thingy still says the cpu is at 63c when playing HL2 though the heatsink is only warm to the touch.