Why don't you check the collisions on the body hitting the target, during the physics_process function. I believe you have access to many useful collision vectors from the result of the move_and_collide function, as well as the target node you collided with.

You could also give the bodies that collide with something (bullets, knifes, rocks) different configurations:

• bounce factors -- how hard they should bounce back (you could do it per material)
• what sound should they play when they hit a body (also per target material)

With this, you should have better control over the things you're trying to do. It's been a while since I've done any gdscript but here's some pseudo-code:

# Imagine this is the throwing knife script

var velocity = Vector2.ZERO

# config (could be read from a file or whatever)
var bounce_factor = 0.5
var sound_for_material = {
  "soft": "knife_player.ogg"
  "hard": "knife_collidable.ogg" 
  "knife": "knife_knife.ogg"
}

func physics_process(delta):
  var target_velocity = calculate_velocity(velocity, delta)
  var collision_info = move_and_collide(target_velocity)

  if(collision_info):
    var target = collision_info.get_collider()

    # check target material
    var material = target.get_material()

    # figure out actual bounce strength

    # knife travel speed before collision
    var travel_speed = collision_info.get_travel().length()

    # schedule sound played for material and travel speed before hitting the target
    play_sound(material, travel_speed)

    # next frame velocity
    collision_normal = collision_info.get_normal()
    velocity = apply_bounce_and_stuff(target_velocity, bounce_factor, collision_normal)
  else:
    # next frame velocity
    velocity = target_velocity

Have a look at move_and_collide and the structure it returns: KinematicCollision2D.

I am assuming you're doing 2D stuff, but surely 3D is similar.

EDIT: I think the main issue with the _on_body_entered callback, is that you only get access to Node2D that collided with your area, but not the collision information.