I wonder if there's a way which allows the modders to replace the default game sounds, specially UI, Some players like to make the sounds matches their gameplay, Like custom sounds for notifications, Popups, Click sounds, Promotion sounds, Increased skill sounds, Ringtone, Emotions sounds..etc.

After a quick research, I didn't find anyone done it before, What make it so difficult to change the sounds?

Wouldn't say it's difficult but wouldn't say it's easy either. First you will have to locate the sound inside one of the core files, extract it then decode it so you can listen and identify it, find a sound in similar length to replace, re-encode the new sound stricty using EA's encoding technique, and create an override using the same instance number as default, quite time-consuming stuff just for a single click or pop, better time spent on a longer sound though, and there is literally hundreds or thousands of UI sounds and its still growing with every pack. It would take a single creator many many years to change every single one of them.

Anyway it's done much the same way as replacing a video or channel. There have been mods that disable certain UI sounds with blank sounds, but the lifetime of the mod isn't guaranteed with patches out nearly every month.