I disagree. Think about all those players who'd buy a unique musical score so that Foundry missions that use it would sound even more... awesome, only to discover that not many such missions even exist yet or ever will at all?!
I think that if Cryptic would go to the trouble of creating unique musical assets (or any art assets) for the sole purpose of being used by Foundry authors in creating missions, the author should pay for the privilege of using the asset if only to ensure the asset actually gets used!
I know for a fact that if Cryptic hired Michael McCann to create music for Foundry missions, I'd probably be interested in buying the pack for use in my missions, but as a player I doubt I'd be as interested since I would not and could not be sure how much use that particular music pack would see.
But the C-Store currently doesn't operate like you describe. It works like this:
E.g. the playable race costumes. The player buys them from the C-Store so that they are able to use them in the game. But they show up in the Foundry for free and unlocked for authors to put in their missions. We don't have to buy them from the C-Store first.
IF there was purchaseable music in the C-Store, then it wouldn't be for the "sole use" of Foundry missions. It would be an extra that players could add to their game's music in the background, e.g. it would randomly play while they were doing regular missions too. It doesn't make money sense to limit it to only appearing in people's Foundry missions - they'd sell it as a feature that is availble throughout the entire game and make it so that they would inevitably hear it in default Cryptic missions, so that even if no authors use it in Foundry, the players get what they paid for.
Also, you get more money from C-Store by having the hundreds to thousands of end-user players buying it rather than marketing it so that only a handful of authors could buy it.
So basically, what I've suggested is the reality and what they currently do. And if they added music packs it would work like that
How about a music player in your Captains ready room or private quarters? There seems to be a bit of repetitiveness of the same music scores in the game though...I don't think people should pay for that. However, that said...no one moans when a new costume set comes out for sale...So why not special themes and scores?