Go to your wow's Data folder's Cache Folder (World of Warcraft\Data\Cache\*)
Everyfile named "SoundCache-patch-#####.MPQ" and on 1 KB is a placeholder
which is the method used to get Custom Textures to work for wow (Doesn't work for Model edits)

Below is a list of SoundCache Placeholders avalible:

{MPQ}

Rename your Custom MPQ's to any of the Files from the list above
and move theme to the Cache folder and select "Yes" to replace the File(s) already in cache folder

Not all Placeholder will work for All versions of Cataclysm.
The higher Number have a chance to work (if you cient also have them!)

Thise Placeholder Works for Current Version of WoW:

{MPQ}

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Here you seen an example how to use this method

You should Notice that the files files listed below have ben replace by Custom MPQ's
SoundCache-patch-13914.MPQ
SoundCache-patch-14007.MPQ
SoundCache-patch-14333.MPQ

If you the Custom MPQ wasn't create corectly, wow will replace it one of it's placeholder files on 1KB

To Remove the MPQ's Either delete them (as wow will re-create the orginal MPQfiles as there no longer is a file by said name)
or move them out of the folder
or replace them with the backup of the orginal Soundcache Patch files