There are a bunch of apps for reading your phone's "native" SMS text messages, but I couldn't find anything for Google Voice, nor could I find any apps that can generically read text in other apps (which I could then apply on Google Voice).
If you have a stock Android phone, or certain more recent versions (I think gingerbread on), then you can use Talkback (either as a separate app or in the Accessibility settings), which does generic text reading on your phone, and should read most things. It does read any notifications which come through, which should include Google Voice (though I'm in the UK so I can't actually test this).
It's designed for blind or partially sighted people, and may not be a perfect fit, but it possibly the best option out there so far.
If you want something that does generic reading, then it MUST be an accessibility app, as only they have permission to read the text from other apps. I don't think Google Voice exposes an external API, which is why you aren't able to find an app which specifically offers this capability.
You can't do this with Google Voice. Apps such as Talkback can only read what you have on your input box. Since Google did not open their API, we can't really make an app for Google Voice. You could always copy the message content to your input box but that kind of defeats the purpose of having the feature of Text-To-Speech.
There's a new app in the market called "Google Voice Text Reader" or "Txt2Speech". This will read your Google voice text messages aloud. Unlike "TalkBack", it won't announce every single touch / event etc that happens on your phone, just your Google Voice messages.
If in your settings for Google Voice, you enable the feature to have your voicemail translated to text, then you have the option to: send an E-mail notifying you when you get a text or voice mail, Then your reader will read your E-mail notifications and you will get your google voice messages and Text read to you.