I've been trying to change the default sounds of steps on the grass. Usually in other Source games (like Half-Life 2) you just type the same name for the sound and place it in the default root for the sound and it will automatically change it.

So I put my four grass sounds (sound/player/footsteps/grass1.wav etc) in the Portal 2 directory (the same place they exist in .vpk file of Portal 2), but the sounds are still the same in-game. The sounds work perfectly fine in Half-Life 2, but doing the same sound change in Portal 2 doesn't work. Tried using snd_updatesoundcache command and still nothing.

Any ideas? Thanks beforehand!

You'd have to override them using a vpk. In portal 2, the files in vpks have the highest priority, and so you need to pack your sounds into a vpk to override them.

Edit: you can also edit your soundscripts in your scripts folder to point to a new sound for the footsteps, but it might mess up for the other person in coop if you are hosting the server.

Thanks a lot