These sounds are by the wonderful New York comedienne Suzanne Rand, charicatured at left by Your's Truly. She's a professional voice-over artist and one half of the classic improv team Monteith and Rand. These are computer alert sounds, so they're short. And her "nuh-uh" is sure to become a classic.
Suzanne has been the voice of the Scotch Brite Sponge Pad, and other animated characters. If you like her alert sounds, hire her to do voice-overs for you! And write to her at:

You can download these MP3 sounds here. (176k)

And here is a recent song by Suzanne (she wrote the words, I did the visuals):