Playsound/stopsound fades, playing from the middle of a track

I've always felt that Java Minecraft's playsound system has been pretty lackluster and extremely primitive.  Basically, all it can do right now is play a sound or stop it.  This works fine for a lot of things, but when trying to create soundscapes or dynamic soundtrack systems, it really falls short.

It would be really helpful to add a way to fade sounds in and out.  For example, if I have a track that's playing in my map and the player crosses some threshold, crossfading the music to fit the different map locations would be much smoother than a jarring cut.

Another example could be the ability to play a track from the middle.  If I have two different intensities of the same track, being able to fade into a higher intensity track at the same timestamp in the track would be extremely helpful.

More examples could be sound sources that can move with an entity, queueing sounds, etc.

Some simple additions to playsound like the ones I've mentioned above would be greatly appreciated by a large number of mapmakers, and would be really helpful in creating awesome sound for maps.