You think you’re getting Fergie, but instead you get a bogus stock tip. Talk about bait and switch.

In the latest pump-and-dump stock-spam scam, spammers are attaching MP3 files to otherwise empty e-mail messages; no subject line, no text in the body of the message. The MP3 files are named after a well-known recording artist, such as Fergie, Carrie Underwood, even Elvis.

But the MP3 files are recordings of a monotone voice telling recipients to buy stock in a little-known company, giving the stock ticker symbol and directing them to read about the company in the news. With pump-and-dump stock spam, spammers blast messages persuading people to buy a penny stock, then once the stock price goes up the spammers sells their shares at a profit.

According to Sophos, which reported this blast today, the recorded voice is randomly altered so that antispam filters can’t detect it.

While acknowledging that the average Internet user isn’t likely to fall for this trick and buy stock in the company, Sophos says companies should consider blocking – or at least quarantining – all inbound messages that have MP3 file attachments.

Pump-and-dump stock spam, which has represented some of the largest spam blasts this year, continue to thrive. Sophos says such spam accounts for roughly 25% of all spam, up from 0.8% in January 2005.