Upon loading the tool, the you can select which bank of sounds to start with, pre-populating a selection of 12 tracks with individual sounds. An 8-bar grid features a clickable series of quarter notes to turn notes on or off for each sound. Tooltips are displayed when you first open the app, and can be accessed again at any time from the "help" question mark at the upper right.
You can swap out any individual sound by loading a new bank from the Instrument panel at the upper right and simply dragging and dropping from the Sounds pane below it onto any existing track. Dropping a new sound on the track will change the sample but preserve the pattern you've already set up in the track, making it easy to audition new sounds for particular parts.
When you're happy with a loop or composition you've created, you can save your work and get a number of options for sharing it: via embed code, MP3 download, forum code, Facebook, Twitter and more. You can also publish your creation to the larger Aviary community for discovery by other users.
Serious musical composers may find the Music Creation tool limiting, but they're not really the target audience here -- the idea is to provide simple and intuitive tools for anyone at any level of musical knowledge to crank out some beats. Considering that only a decade ago the tools of musical production still set back would-be composers thousands if not tens of thousands of dollars, bringing simple point-and-click loop creation tools into the web browser is still an impressive milestone in the overall democratization of musical creation.
Moreover, the sample sounds are decent and the sharing tools are a nice value-add here. Aviary has also chosen to release the tool without commercial restrictions under a Creative Commons Attribution license, meaning that the provided instrument library can be used to create compositions for any purpose, even commercial usage.
You can check out our own chicken scratching test loop for an idea of what you can use the Music Creation web app to generate. If you get a chance to sample the tool, be sure to let us know your impressions in the comments.