When the thing being offered for sale is viewed as a product, then infinite supply plays against any demand based market, just as you’ve pointed out. But if viewed as a service, a service providing timely and on-demand distribution, then higher prices for “better service” could make a lot of sense, especially if buyers of “premimum content” (ie music that’s in high demand (for whatever reason (recent releases, or other culturally defined high-demand times)) are given priority for the ability to access it. I would be willing to pay a premimum price to download something with a higher priority over people who have paid less (various computer game websites have done this for a while (fileplanet.com, fileshack.com), you pay and you get to go to the front of the line). But the thing here to make sure it is billed/advertised as an on-demand service that is being paid for, not the “product” itself. Someone who’s been searching for the out of print Shatner album The Transformed Man, which won’t have a high current demand, won’t mind waiting a bit to have access to it, OR won’t mind paying a larger price to ensure his access to it.
In other words, while the supply of the product itself is infinite, the supply of the distribution channel is not (bandwidth and databases and computers and storage all cost money), and that’s what you should be paying for. People are usually more comfortable playing for things like this on a subscription basis though, rather than a per-access.