Interesting concept. Played a little bit with it, I guessed wrong all but one (which probably says a lot either about the music or me ).
This will be specially interesting for more complex compositions. Many of the samples in the site sound like fanfares or musicalized sound effects, but more complex arrangements can vary a lot from a range of meanings / emotions. It's also a great source of feedback (even if indirect) for a music composer - this applies to vgm composers but general as well
I don't really like the flow of the app when you are not registered (I don't know if it's the same when you are registered). Too many clicks. IMO, the site could be much better if I didn't have to click that much, and the site would allow me to listen and guess in the same page, and maybe reload a new melody in the same page. User interaction is as important as the content itself, so use this opportunity to engage the user and make it better.
I'll keep playing with it a little more, thanks for sharing!