i'm thinking about how you'd do this,
and the picture in my mind is of a single 2 second long clarinet note with a hihat being hit after about 1 second.
so, if you were going to stretch or compress that, you'd want to preserve the parts with the most 'colour' ...which i would guess would be the attack of the clarinet and then especially the attack of the hihat.
Maybe a bit too naive approach, but what about chopping up the audio in small fragments, and discarts fragments that have about the same spectral footprint as their neighbours ? This will leave the dynamic parts intact and leaves out the 'boring' bits.
Ico