Actually, sounds can still be muted and replaced! It works differently since patch 8.2, explained here in my guide using the add-on OldGunSounds. ➴ How to Get CUSTOM Gun/Bow Sounds Back in Shadowlands
If it is only muting you want, try https://www.curseforge.com/wow/addons/mutesoundfile or another muting add-on from Curse/Twitch. If you have any questions, let me know!