I signed up to NetAudioAds - which was supposed to be live in February 2008, but got the following message:-
"We are running about 100,000 ads a day, spread randomly across member sitesStill it has potential if you get loads of visitors as it seems you can use it in addition to Adsense.
as we check out all of our reporting and financial processes.
We expect to be testing ad placements throughout the next two to three weeks
Thank you for your patience."
It may annoy users somewhat, but for me that is not a big problem, as most users dont stick around long anyway after they have gotten whatever free image they were looking for!