This is a fairly new tactic used by spammers. If you frequently download MP3 files (hopefully only those you have permission to download) sooner or later you might unknowingly download a file that doesn’t contain music but instead has a recorded message.
The message is computer generated, so it doesn’t sound human at all. And it tells you to invest in particular shares, called “penny stocks”. Obviously you’d be crazy to take financial advice from an audio spam you accidentally downloaded (in other words, delete the file and don’t buy their shares).
There isn’t much you can do at this stage to avoid it, other than taking care to download audio MP3 files from reputable (and legal) sources. I expect antivirus or antispam filters to quickly start checking audio files as well as the usual emails.
For now it’s being called “MP3 spam” by the media, we’ll see how this one evolves.