Crafty spammers overnight launched a new wave of pump-and-dump scams, this time delivering junk mail that includes audio attachments encouraging recipients to buy a penny stock.

Experts today said the new MP3 spam tactic is creative, but it seems to be a natural progression following runs of image, PDF and Excel junk mail earlier this year.

Anti-spam outfits reported Storm Worm-driven MP3 spam runs of about 10,000 per hour, accounting for roughly seven to 10 per cent of all unwanted mail in the past 18 hours.

"It was almost expected in the natural order of spam," Paul Wood, a senior researcher for MessageLabs, said. "They're just looking for the next big thing, and they've probably found it."

In most cases, the junk mail arrives without text in the body or subject line, and includes an MP3 attachment that employs social engineering to appear like a trusted file. Depending on the message, the file name might be "bspears," "smashingpumpkins," "weddingsong" or "coolringtone."

In actuality, the files contain a recorded 30-second synthetic voice message from a woman who tries to persuade listeners to purchase stock in Exit Only Inc., which does business as Text4Cars.com.

The company, whose customers mostly live in Canada, is a thinly-traded stock that is listed as EXTO on the Pink Sheets. This type of business is commonly used in pump-and-dump scams, where even small volumes can move a stock several percentage points.

Text4Cars.com tries to match car buyers and sellers through text messaging, CEO David Dion said. He said he runs a legitimate company and is not trying to get rich quick off a spam scam.

"Why someone is targeting me, I have no idea," he said. "I wish they'd leave my company alone."