Here is a cool extension that will let you change the facebook chat sound alerts to a few sexy sounds. ( yea, really, there some sexy sounds 
)
Once installed open your facebook and see the new Speaker icon added next to the chat row, click and choose from one of many cool and fun sounds. When someone writes you a message the newly selected sounds will play.
Give it a try ! Install it from here.
Please do NOT download this. Whoever said this was a good idea is either an idiot, ass, a member of the company, or most likely all three. It gives you sounds, but also sends to every single person that you chat with, “Hey! I got these awesome chat sounds at (website) for free! 8-)”. Yeah…don’t do it. Luckily, it can be disabled and uninstalled on Firefox by going to the extensions options under tools -> add-ons, and then finding “Face Sounds”. Good luck, and once again, do NOT download.
oh i see……. so it is -> VIRAL
*like what viruses do..
thnx for the tip Doc! XD