There's an implicit assumption of "more than one unit" in a market. One of anything is a curiosity, not amenable to economic analysis. In this particular market, popular music, where the issue under discussion is massive copying, it's downright disingenuous to do the "sunk costs are marginal cost of the first unit" schtick. Marginal cost gets calculated without sunk costs.
It says right there in the link you provided that the marginal cost is the change in total cost that arises when the quantity produced changes by one unit.
We're discussing the mass entertainment market, where total costs are dominated by fixed costs. Do you seriously think it's disingenuous to take fixed costs into account? I'd say it's far more disingenuous to just ignore the bulk of your expenditure.
