The obvious revenue model for Ambisonia would be to offer paid download. Buy a track for 90c (downloaded).
This is actually very difficult because of the amount of money involved. Online purchases are usually done with credit cards, and credit card purchases are not viable for anythinig under $10 (because of the charges on credit cards). One option is restricting purchases to PayPal but this is limited (and many people really dislike PayPal).
The other option/solution is to sell ‘credits’. This is how iTunes / Amie Street and others do it. Buy a minimum of $10 worth of credits (for example).. and then spend that as you like.
The problem with this model is there is a significant barrier to entry. Who will spend $10 to buy their first 90c track?
Other than that, building the (secure software) infrastructure to manage credits will be very involved. So, whilst this is still a long term goal, there has to be an other revenue model _before_ this kind of thing is implemented.