How is this accomplished? Do you have each note of musical instruments available as a unique sound in the game?
You have a bunch of different sound items that for instance activate when something is near it, which contain a sample that you can also modify a bunch of parameters for. So you can create various monkey sounds using the monkey sample, and ditto with individual instruments.
@Robert1: the story mode contains about 50-60 levels if I remember correctly, playable with up to 4 players, with leaderboards for each and every one of them, etc. Should be a more than capable platformer in that respect!
And remember that many of the levels you've been playing online have been created in 5-15 hours, typically, by a limited group of people. Imagine what things will be like when people have been able to work on their levels for a few months, have learnt the strengths and weaknesses of the tool, learn from each other what kind of things you can do ... I think you'll end up being amazed. And it will be interesting to see how many other developer teams will create a level of their own, or create one for promotion purposes of their own game, etc.
They left in the key triggers so you can see how it works. I think that it could be possible to extend this idea and make a guitar hero like level where you have to move about to trigger the note and where the trigger releases points bubbles for each of the successfully triggered note.