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I've been thinking and the idea sounds good.I think the idea is bad.
1. Shoveling ads down your visitors throats is a sure way to drive them away. Consider how Google does it - they display ads unobtrusively, in a way that helps you. If they were forcing you to read through the ads before showing the search results, do you think they'd be where they are now ?
2. Consider the artists. They need to be desperate to have their work preceded by 20 seconds of diaper commercials. I realize you can play relevant ads, but these might be very hard to come by. There's lot more Poker and insurance ads than those of high-end speakerphones and such.
3. In the light of #2 - desperate artists aren't probably be the source of an awesome material, so the quality of your site content is probably going to be low. Additionally the artist churn rate is probably going to be high and you will end up with stale material.
In general an easy way to validate an idea is to imagine that you are past the initial growth stage and now are at the stable operational state. Who are your contributors, users and advertisers ? Is their combination realistic and stable ? In your case it's neither, so the idea is not viable.