There's more than meets the eye with Song Prototyper, in my view, making it a welcome surprise that the app is totally free. Here's the very minimalist description in the Windows Phone Store:
Song Prototyper makes it quick and easy to turn those music melodies, riffs, and licks into a song you can shape. Helps you organize and record audio with full SkyDrive functionality.
Expanding on this, let's take a look at Song Prototyper in use for real on Windows Phone 8:
Songs can be added as needed, and you can work on many songs at once - after all, that's the whole point of an organisational tool like this; (right) within a song, you can add as many sections as you like, with title and notes. Each section's audio can then be played in sequence if needed, in the right order for the song.
A section also has an associated time signature and tempo, set from sliders if needed. You can then record your idea for each in time with the visual time meter. In my case, I tend to hum it or play it on the guitar or piano.
Full SkyDrive integration means that your recorded snippets and all the song structures can be saved (in case of disaster, such as getting mown down by a bus before you can get that masterpiece hook recorded!); (right) you can also open up projects directly from SkyDrive, which is a nice touch.
If you're the sort of musician who gets inspired in odd moments and then worries about writing it all down or remembering 'how it went' then this could be a very useful little tool indeed. You can download Song Prototyper here for free in the Store.