Stock spammers have abandoned PDF files and image spam with a new campaign featuring MP3 files.

The audio files pose as music from stars such as Elvis Presley and Fergie, but actually contain a monotone voice encouraging punters to invest in an obscure Canadian company.

The MP3 files are being widely spammed in emails that often contain no subject line or message body, net security firm Sophos reports. Some of the filenames used include hurricanechris.mp3, allforone.mp3, carrieunderwood.mp3, elvis.mp3, baby.mp3, fergie.mp3, and bbrown.mp3.

The female (apparently British) voice on the MP3 file, recorded at low bit-rate and randomly altered to avoid detection by anti-spam filters, seeks to attract interest in Exit Only, a Canadian firm that runs a website marketplace for new and used motors. Sophos has posted a 30 second example of the stock spam MP3 file here.

Quite who would respond to such an unsolicited pitch is hard to fathom. Anti-virus firms advise users to block MP3 files in email.

The Exit Only spam campaign is the latest variant in pump-and-dump scams, a type of fraud that now accounts for 25 per cent of all junk emails compared to just 0.8 per cent in January 2005, according to Sophos.

Pump-and-dump scams are email campaigns that seek to encourage armchair investors to sink their cash into particular firms' stock.

The goal is to quickly inflate interest in low-value stock with bogus insider info in order to ramp up share prices and sell at a profit before the inevitable crash and burn. Meanwhile, those duped are left holding possibly worthless shares.