Check out your game's analytics, there you can have some insights about from where people are reaching your game's page. But don't feel afraid to charge for your work, you will find out people can be very willed to pay, if you set it. You can think this way:
OK people are downloading and playing my game, so the concept and the actual landing part I'm doing well, so the game's production is validated, now let's see if we can validate its price and value as well
And start to test your product. Maybe start with:
- Game
- Game + Soundtrack
This way you can at least make people compare between two of your products to choose if they gonna spend their money, instead of one of your products and another studio's product. For instance, if you have only the very Game being charged, people can think:
OK this game is $X and this other game is $Y, so let's check which one of these I would like to play more, based on my personal interest
While when showing 2 or 3 of your products they will be more likely to think:
Hmm, the game by its own is $X, so I can play it and have fun, but game + ost is $X+1, why would I want the ost if I can play the game, wait... game + ost + wallpaper is just $0.5 above game + ost, hmm maybe the ost can be nice to listen while I workout...
But notice that all of these comparisons are between your products, so...in any of the cases is a win situation :)
Don't be afraid to charge, people that get the game for free can be considered "rewarded" for being the first ones to trying your game, so is fair that they got it for free.