I signed up to this adnetwork that lets you play audio ads and pays you for it. It only just recently launched and it is not fully active. At the moment they play 100,000 ads a day across the network. I only played 20 ads but received $0.07 for it. I think that's pretty good conversion. There are also 2 levels of referals where you get paid for. So if you are interested then feel free to sign up via this link.