Hello! To change the default walking sound you must obtain a LocalScript called RbxCharacterSounds
To obtain it you must Enter a test session in studio, and in the Explorer under StarterPlayer → StarterPlayerScripts you will find a script called RbxCharacterSounds (copy the script), Exit the test session and paste the script under StarterPlayer → StarterPlayerScripts. On Line 38 Change the SoundId to game.ReplicatedStorage.Sounds.Walk.SoundId!