A German scientist named Hartwig Hausdorf* thinks the sound is significant because it is first contact with an alien culture:
"It seems almost as if someone has reprogrammed or hijacked the probe," he told German newspaper Bild. "Thus perhaps we do not yet know the whole truth."
Nasa's explanation is rather more prosaic. It blames a software problem with the craft's flight data system. Voyager 2 has been given instructions to transmit only information about its own status while scientists try to get to the bottom of the mystery.
* Not related to aliens or NASA, but Hartwig Hausdorf should have been Jerry Lewis's name in The Day the Clown Cried.