The Storm Worm botnet network may be shrinking in size, but it has managed to send out 15 million of those annoying audio spam messages in October, according to anti-spam vendor MessageLabs.

It's hard to believe that the Storm messages were effective. Recipients had to first click on an attachment -- usually given a misleading name such as beatles.mp3 or Britney.mp3 -- to hear the stock pitch, which featured a warbly robotic woman advising people to invest in online car seller, Exit Only.

This kind of scam, called "pump and dump," tries to nudge up the price of penny stocks by a cent or two, giving the spammers a way to make a quick buck by selling the stock before it crashes. Spammers have been delivering their messages in different formats, including PDF and Excel files, over the past few years as part of a cat-and-mouse game with spam blockers. This move to MP3 spam is the latest development in this battle, observers say.

Spam watchers say that pump-and-dump schemes are the hottest and most lucrative area for spammers today.

The spam run began on Oct. 17, and lasted about 36 hours, using infected computers in the Storm Worm network to send out the mails, MessageLabs said in a statement released Tuesday. The spam sounded strange and warbly because the voice in the message was "synthesized using a very low compression rate of 16KHz to keep the overall file size small, at around 50KB, to avoid detection," the company said.

Storm is thought to have landed on as many as 15 million PCs over the past year, but recently its network of infected PCs has been shrinking. University of California, San Diego, researchers recently pegged it at about 160,000 computers, only 20,000 of which are accessible at any one time.

Exit Only said it was not involved in sending the spam. Its stock was trading around $0.41 on Oct. 18, the day after the Storm spam started. On Tuesday it closed at $0.20.