Well, you can accomplish deferred decoding thru setting 'preload sounds' to 'false', and using the 'preload audio' action to load in only the sounds you need, avoiding the delay that occurs if you try to play an un-decoded sound. Personally, I would consider this a 'memory management action'.
However, we have no tools or options for unloading decoded audio. As you said, we should at least be able to unload audio at the end of layouts.
Personally, this hasn't been a problem for me, but my mobile projects have been on a small scale, with minimal audio. I could see it being a problem for a more ambitious/audio-rich project. Case in point: I became aware of this from posts by Tinimations about Klang, which isn't even a mobile game.
Anyway, returning to the thread subject, I think the sluggishness of decoding is an even bigger problem for most mobile games...who wants to wait a minute or more for a simple game to load?