This addon will mute our verbose Syrenne champion. You do -not- drop this addon into Interface! You must unzip this directly into your WoW directory (where you have the WTF, Data, Interface and etc folders).
This is a one time thing I did for myself and placed available for others. I am not interested in doing it for other bodyguards, but I can explain how I did it so my footsteps can be followed:
First go to wowhead.com. Then navigate through Database, Sounds, to NPC Combat. There I searched for Syrenne and got all her sound files. Then I looked at them one by one. If your browser plays the files all the better, but mine didn't so I had to click on the script and paste it on WoW to hear them.
Once I had discovered what each file sounded like and assembled the culprits I had to create substitutes. As an example one of the files is called VO_72_Nighthuntress_Syrenne_22_F so what I did was create a file and name it VO_72_Nighthuntress_Syrenne_22_F.ogg.
I had some trial and error to discover where to place it and how to name the folder. This is important. Simply naming the folder Syrenne did not work, and neither did NighthuntressSyrenne, but Nighthuntress_Syrenne did. You might have to trial and error as well always remembering you need to completely exit WoW to test the changes.
In this case it is Sound/Creature/Nighthuntress_Syrenne.
You won't need to create this for the addon I've provided since it already creates the path.
Good luck in muting your bodyguards!