Uhm, there was, you had to click the little icon by the plurk update bar...

but I havent had the noise in months, I didn't even know it still existed... It's not in the preferences?

I looked in my profile and my account and didn't see it.

Found it. Google is great! LOL! When you get the little new message box in the bottom left there is a speaker icon. Click to turn off.

really.. I don't have that icon, ever

weirrrrd