one local club member put a pair of whistles on the wingtips of a Gremlin flying wing for R/C combat. He placed them right at the thickest part of the airfoil. Needed to go in a vertical power-off (engine at idle) dive to hear them over the engine. [:'(] He moved one forward (lost the other when a mid-air sent it flyiing int the brush) and suddenly it was audible any time the plane was above 2/3 throttle in level flight.
Experiment a bit with placement. You will find a location that give the maximum sound at the flight condition that you want it to be loud.
