The penny stocks are being promoted with
MP3 voice messages with deceptive sounding names like 'santana.mp3', 'bspears.mp3' or 'coolringtone.mp3'.
The messages are much larger than image spam and PDF spam, and can reach 147KB.
For the most part the message contents are empty with subject headers containing just 'Fwd:' or 'Re:' or the name of the file attachment.
File names have been categorised as 'emotional' (dadsong.mp3, oursong.mp3, weddingsong.mp3), well-known artists and songs (santana.mp3, sayyousayme.mp3, smashingpumpkins.mp3), or 'sounds' that people might want to listen to (answeringmachine.mp3, coolringtone.mp3, listentothis.mp3).
Danny Jenkins, MXSweep chief technology officer and founder, warned that spammers are using new techniques every day to penetrate spam filters.
"As their methods become more and more advanced, traditional keyword and Bayesian filters are becoming less effective in the global fight against spam," he said.
"The vast majority of businesses are already blocking audio files or will have no issues implementing corporate policies to block audio files from arriving by email into the user's inbox.
"Although these emails now account for eight percent of current traffic they consume up to 55 percent of email bandwidth use, which is a huge additional cost in business terms."