These will sound “tinny” if played on a smartphone built-in speaker because it can’t play the low frequencies. Built-in computer speakers will be slightly more accurate. Standalone speakers or a home sound system will replicate the sounds most accurately.
Starting with the smartphone is one way of making these kinds of sounds less scary. That may or may not be necessary for Biscuit.
Knock inner door 1
Knock inner door 2 (closet)
Knock inner door 3 resonant
Knock front door