20-30 sounds right. And I didn't even get everything. What really gets me about this game is the atmosphere. It elicited a lot of emotion too. Some things in the game scared the piss outta me. But you're completely right when you say that the soundtrack is worth it. It fits the mood well. The singing aspect is really fun, to the point that I purposefully avoided using the shortcut keys (1-9).
As for replay... not so much. You'd know where everything is, and it's too long to speedrun. There are two endings, but it's more of an addition to the original ending. So getting the 'better' one shows the other. While it has nearly 0 replay, there is a sequel coming. And there might be user created maps, but I haven't really looked into that.
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