Also, what is a "premier track?"
If a user (currently US only) is listening to their stream and they start playing a track a premier member has decided to monetise an ad will play in front of it. There are some other factors that will influence ad frequency obviously, but that is the general behaviour.
Just because ads are a horrible way to monetise a web endeavour.
Subscriptions and rev shares are generally far less friction than charging up an account and explicitly giving a micropayment to an artist. I think SoundCloud already is a "marketplace" where independent artists can function without labels (if they want to do all the other work that labels cover), just that currently all transactions happen outside of the platform. That said, I don't have any more info about how a premium subscription for users would function.
If you must have ads then you should have content restrictions like google did with Adwords. Allow no music, no sound effects, no time effects, just a vocal sales pitch with ordinary dynamic range and -12dB peak loudness. The extremely limited options for Adwords presentation are what makes them bearable, in contrast to the hideous animated mess of typical internet banner ads.
This is a one-time opportunity to establish standards that will support rather than undermine your brand. If you let advertisers have free reaign over what to do with their slots then you'll get disrupted in turn. I already hesitate to embed Soundcloud clips because the embeds don't have any volume control and the play/volume controls on the site have been moved to the opposite end of the screen from everything else, a depressing example of a user-hostile 'dark pattern'.
"Carefully-controlled? The ads will only be seen and heard in the US for now, on SoundCloud’s own website and mobile apps – not in tracks embedded elsewhere on the web and/or listened to elsewhere in the world."