Yes, there is a critical point where the separation will occur regarless but before that its a matter of the motion of the ambient fluid (air). Given that a propeller is first started when the ambient fluid is at zero velocity the intial acceleration of the fluid mass could take it near compressible conditions, but as soon as the fluid passes the transient stage into the steady state condition the velocity becomes non-zero and the compressibility effect is reduced. Thats when the sound stops and the thrust increases. This effect is more noticeable when there is a duct around the propeller (ducted fans and wind tunnels).