Some of the default sounds are... let's say no as good as they could be. So lets talk about alternative ones.
Form Mito551's soundpack I would use
default_dig_choppy
default_dig_immediate
default_grass_footstep.1
default_place_node.1
default_place_node.2
default_place_node.3
From Jordans
default_grass_footstep.2
default_grass_footstep.3
and that one: http://ompldr.org/vZ3diMw/default_dig_crumbly.3.ogg (this is a combination of 1 and 2)