Hello!
Have you replaced the walking sound completely? If you do have the original sound file in there then it with contradict each other. Also, make sure that you do keep a backup of the original just in case. :-)
Also, if you click it on your computer through the files (FeralHeart>Media>Sounds) does it still play? There could be an issue with the conversion from keyboard to computer to proper sound file for FH. It should be a .wav and not a .mp3 or .mp4.
I hope this helps!
Other than what Spicy already said, I wonder if the sound length has anything to do with it. When I open up walk_grass, I noticed that the sound is only roughly a second long. How long is the sound you tried adding?