Every so often, someone will say they play WoW with the sound off, and listen to music or TV while they play. I don't really get this... I can do that while I'm mindlessly playing an alt, but I really need sound while raiding or doing heroics.
Part of it is that sound adds a subconscious level of immersion. Playing with the sound off just feels detached to me. Plus it's rather hard to focus on a raid while Mythbusters is on TV! But I also feel that sound is an important part of the user interface, and I think that its use should be encouraged and enhanced, and not muted.
There are many built-in sound cues in the game. You can hear a sound effect when you get clearcasting, or when your attack is parried, or when your Prayer of Mending bounces. These are a good start towards building situational awareness, but like the rest of the default user interface, leaves something to be desired. Plus there's the fact that all the ambient sound from players near you and the world all combine into a cacophony of spell effects and grunts and clangs of steel!
So, we extend the role of sound, just as we extend the default UI. Our mods can play sound effects. This is ideal because we can choose which events to single out with an auditory notice, and we can pick any sound effect we like to go with each. Blizzard created a lot of great sound effects, but sometimes there's a subtle sound effect for what is actually a very important event, like a void zone spawning under your feet! When we configure mods to play our own sounds, we can pick distinct and memorable sound effects to go with important procs or debuffs.
A well-known recent example is how Deadly Boss Mods yells "Run away little girl!" when Sartharion summons a lava wall. Interesting that they chose to play a (very recognizable) sound clip when this happens, in addition to showing a timer bar. On the other end of the mod spectrum we have Comix, a mod that plays the old Batman "POW!" sound effects when you crit. Silly yes, but very evocative, and it also plays other sound effects when you get important buffs, and sings "Extra life!!" when you get a battle rez. If the author added funny sound clips for getting Grobbulus's disease, or the different charges on Thaddius, perhaps it would be a required raiding mod!
A personal example is that when learning Sarth 2D and 3D, I had trouble avoiding the Shadow Fissure void zones. With 2 drakes up, between the extra healing demand, the fire walls, and the adds, it seemed I often forgot to look at my character to see if I was standing in a zone. So what I did is write a mod that plays a silly sound effect each time a Shadow Fissure is cast. Once I had this enabled, I was able to make the association: hear sound, look down to see if I'm in a fissure. It has been a great help; I still get healer's tunnel vision at times, but now I have auditory cues for fire walls and void zones.
I think these example show how sound can be used effectively in mods. Sounds are transient, but powerful. Sounds can act as an interrupt, an alarm. This is why fire alarms use sound, why ambulances use sirens, and it's also why someone calling an important event on Ventrilo is often far better than everyone having one more timer bar in their already-cluttered UI's. We are visual creatures, but a full raid UI is already overtaxing on the eyes. Best to use sound as much as we can, to let the ears help us parse information. To only use sound effects for ambience is to miss their power in alerting us to important events.