Sound Addons

A Peon's Burden

Plays the peon "work complete" sound each time you complete all the objectives for a quest. Optionally, you can also enable the addon to play the "work work" sound each time you advance a quest objective.
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CritSound

Plays a sound when you perform a critical hit or healing.
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GladiatorlosSA2

This is a special addon, it has no interface, but it can speak. It is SOUND ALERT addon leads you to become GLADIATOR, so having the name GladiatorlosSA. This addon will alert spells using customized voice. For example, when you are in BG or arena, an enemy uses his pvp trinket, a voice of "Trinket" will come out. It can alert many spells(100+), each spell can be selected on or off. The voice sets made by TTS(text-to-speech) is Neospeech US English female Julie. The default voice changes by your locale.
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Hear Kitty

Hear Kitty plays soothing and distinct orchestral sounds as you gain and spend your combat resources.  It's like having your own theme music!  Rogues and feral druids hear tones as they gain and spend combo points, enhancement shamans hear their maelstrom weapon charges, and paladins hear their holy power build up.  When you hear the final note played, you'll know it's time for a finishing move, and you won't have to watch for red combo dots or buff stacks anymore.
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Leatrix Sound Kit

Leatrix Sound Kit allows you to replace a variety of game sounds. Silence combat racial sounds, Silence combat vocal sounds, Silence interface sounds, Silence NPC sounds, Silence spell sounds, Replace footstep sounds
wowi

Mute Annoying WoW Sounds

Mutes some of the annoying sounds of WoW.
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QuestChimes

Simple addon which provides an audible feedback when completing a step of an objective (i.e killing 1 of 10 mobs) or completing a quest.
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QuestNoise

This addon plays sounds whenever you make progress on a quest. It plays a different sound for completing the quest, completing only a single objective of the quest, or making progress on an objective. Unlike the other addons I've found that provide the sound feature of GuestGuru, this addon does not require or use LibQuixote (which does many quest-related things that we don't use or need).
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Quest Progress Sound

Tracks quest progress and playing sound files when a part of a quest is finished or a quest finished at all.
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Work_Complete

Quest Progress messaging and sound. Plays "Ready to Work" and "Work Complete" audio messages. Additionally, it will send the quest status to party chat if you are in a group.
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