Also called PPP (Pay Per Play), this advertising method was introduce by Net Audio Ads. the concept is pretty simple: play a small audio advertising (usually 5 seconds) every time a visitor enters into your website. The user should not be able to stop it, creating a 100% conversion rate based on unique visitors.
The company is still rolling tests, but some users are reporting to get from a $4 to a $6 CPM. Regardless of the pay rate, though, this is a very intrusive form of advertising, so think twice before using it.