Sound would be cool...but we'd need some more complicated storage of memory if we wanted to properly implement it...some sort of Memloclist so in a free memory location we could store a 'chain' of information instead of just 1 thing...
I'm still though routing for scent, and chemicals etc. (because if we added scent we'd need chemicals and nutrients around the map as well)
Offtopic: I just thought surely decayed corpses (so ones no longer on the map) should have some sort of energy boost to nearby plants?