So I gagged her.
In case anyone else is interested in this sort of thing, here's the modification I created.
The zip contains a readme, which I'm also including here for convenience.
A Turtle Has Made It to the Water Quietly
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Description
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This is a simple modification which silences Scrollsage Nola's voiceover line:
"A turtle has made it to the water!" during the Tortollan Seekers' world quest
"Beachhead". It does not hide the onscreen message box nor the line of text
which is added to the chat window.
Installation
------------
Unpack the "Sound" folder from this archive into your main World of Warcraft
folder. If you can see "World of Warcraft Launcher.exe" and "Wow.exe" files,
then you are in the right place.
Do *not* unpack it into your "Interface\Addons" folder! This is not that kind
of an addon.
Removal
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Delete the "vo_801_scrollsage_nola_33_f.ogg" file from your World of Warcraft
"Sound\Creature\scrollsage_nola" folder. You can also delete the folders up to
and including "Sound" unless you have additional sound overrides there.