I'm no expert, but one observation I had is podcasts seem to follow this model. Most podcast ads I hear are read by the hosts themselves, though a small number splice in targeted advertiser-provided audio (eg a UK soccer podcast will play me local Canadian ads).

For the podcasts that follow the former model, they don't seem to rely on timeslots (i.e. payment just for ad being shown), though some may be involved. Instead, it seems they rely on coupon codes to measure conversions. I assume the podcast gets paid per coupon code redeemed. Barring show sponsors, in my experience ever single podcast ad read by a host will use a coupon code.