The only way I can find would require you to write a bit of Python code.
This is the site that provides a module for Python
This page gives you details on how to set a message to read. Look for the Mark function under the Message section.
I would write the script for you but I do not know enough Python to make an attempt.
Hope that helps some.
EDIT: shouldn't belittle my abilities. Here is a script to set everything to read.
from googlevoice import Voice,util
voice = Voice()
voice.login('YOUR USERNAME', 'YOUR PASSWORD')
while True :
folder = voice.search('is:unread')
if folder.totalSize <= 0 :
for message in folder.messages:
UPDATE - gygooglevoice needs a change in its settings page to work properly, otherwise you will get a login error. Change needed is referenced here: http://code.google.com/p/pygooglevoice/issues/detail?id=64#c4 (just update the Login URL)