Loading the magazine:

1. Open the inventory.
2. Put the magazine into your hands.
3. Drag the bullet-stack onto the magazine in your hands and release it on the combine option.
4. Hold the left mouse button to reload the magazine with the bullet-stack.
5. It will now play the sound effect of punching instead of a bullet loading sound effect.

Unloading the magazine:

1. Open the inventory.
2. Put the magazine into your hands.
3. Hold left mouse button to empty the magazine.
4. It will now play the sound effect of melee attacking with a sharp weapon instead of a bullet unloading sound effect.

Stress test build version: 0.63.146661.
I assume it's just placeholder sound effects and not a bug (I thought there was sound effects for these two actions already, my bad).