No, it only speaks the result if there is single answer to say. If you ask it what is 15% of 51 it will speak the result. But if you ask a broad question (what is quantuum mechanics) it will just list all results.

I tried 15% of 51 and also queries like what is the population of Tokyo similar to the demo. It does not seem to want to speak the results back.

Which phone do you have? I suspect this could be disabled on older iPhones. I have a 4 and I don't get any voice feedback.

I tried it on iPhone 4 as well and it does speak some answers. Touch the "i" button on the left of the red microphone button and make sure "Speak answers back" at the bottom is turned on.

Interesting.

I can the "i" button in that video, but my app does not have it, and I don't get voice answers.

Edit: Switching to English (US) does give vocal answers, but then it doesn't seem to understand my South African accent anymore.

I am on the iPhone 5

For some reason I had English (Australian) active in the Google language settings even though I don't live there. It started reading the answers when I switched that to English (US).

Yes, that did the trick! English (US).

Under "Voice Search" in settings try changing the default language to US English.