Despite having distinct footstep sounds in the game files in addition to unique sound events for these footstep sounds, Silverfish do not use any footstep sounds at all in-game. 

no_silverfish_footsteps.mp4

minecraft-sounds-mob-silverfish-step1.ogg

minecraft-sounds-mob-silverfish-step2.ogg

minecraft-sounds-mob-silverfish-step3.ogg

minecraft-sounds-mob-silverfish-step4.ogg

Here is a video demonstrating the sound event working in-game:

sound.mp4