Google Voice Voicemail is not Publicly searchable.

I can search my GMail on the web as long as I am logged into my account, do I worry about my mail being public, yes and no. Yes because I want to keep my email private and no because I know my data is private with Google unless I share them with others. That goes for all other information that I create and use on Google. I believe they are not searchable, yes voicemail included.

There was a big questions about Google Voice and it' voicemail privacy yesterday and I was too busy to respond. But Google Voice blog has clarified information for all of us.

Google Voice lets you access your voicemails online from your inbox. Your account is password-protected, like any other Google service, and its content cannot be read by anyone unless you choose to share the information in your account.

You can read and search as much as possible, but Google Voice Voicemail is not Publicly searchable. Follow the link if you need further conformation.
About Voicemail and Privacy

About Voicemail and Privacy