If the audio node is child of the projectile it will most likely be deleted with it when you call queue_free, so it prob starts playing the sound and then gets deleted.
Multiple solutions for this, you can prob make the sprite of the projectile invisible to make it seem like the projectile got deleted, and then connect the signal of the audio node finished() to call the queue_free once the sound is done.
#cant hear bullet hit sound
stiff brook
fair bluff
yep that was it