As I said, it's written there, you just need to read, and read it correctly not like "It says it can mute sounds"
What addons CANNOT do
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Change sounds of skills, weapons, armor, procs, pets, mounts, houses, music, NPCs and other non-UI related things
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"non-UI related things" implicilty tells you we can only mute UI sounds, what the thread you have linked also tells you via the sound name.
UI sounds = click sound, ultimate ready sound, book page turned sound, etc.
But unfortunately (or maybe good too at the same time!) we cannot change any NPC or pets or fight or similar sounds as the "What addons CANNOT do" tells you.
Addons that mute some NPCs just do not mute the special sound but check the chat for texts at the NPC channel (like that smithing guy in Riften telling you that Idesa is sooo distracted..." and then turn down the total volume for a short tiem to "mute them". So this is a workaround only.
But pets do not write to chat as they make a noise or play a sound so this is not possible.
If you want to mute UI sounds check out "FCO ChangeStuff". It got a list at the bottom where you can move the sounds you want to move from left (played) to right (muted) and then these chosen sound should be muted. Click the sounds at the left to play and hear a sample. Some of the sounds exist but do not play anything so this is norma, just in case you wonder why some are silent.