use $routeParams to get object key
bug: tracks were leaking between objects

when changing to an object with fewer tracks, it only updated `sources`
for as many tracks as the new object had.
`sources[buffers.length..oldSources.length]` weren't reset, so they
remained in `sources` even though they came from the previous object.
Clear them out with a `splice`.
autoplay
Save the current mix