As Luke slashes an enemy with his lightsaber and he falls into the Sarlacc pit. It can also be heard again a second time as Luke slashes another enemy into the pit soon after, but it is barely audible. Additionally, in the Special Edition, a Wilhelm can be heard during one of the huge celebration scenes, on Coruscant, after the Death Star is destroyed (an Imperial Stormtrooper is crowd surfing and the Wilhelm is heard as he passes to the right of the film frame; since this is a celebration scene rather than a fight scene, the Wilhelm was presumably included as a joke).
The Wilhelm can also be heard as an Ewok is killed while taking cover from the Imperial Walker turret fire.
The Imperial officer (played by Ben Burtt) in the bunker who says "Freeze!" and gets knocked into the generator room by a thrown satchel. When he falls over the edge, he attempts to emulate the Wilhelm scream, which he made famous.