I've been playing this game since launch, and I remember certain audio files used to work. Specifically, certain mentor lines such as:
"You need a training room to train creatures."
"You need a combat pit to advance your creature level."
"You need a casino so your creatures can relax."

I know for a fact that they used to work, because I can remember the lines and the exact tone used. However at some point, I assume after one of the new patches, many of the automatic voice lines and audio is broken. I assume it's because of an official patch because I own both retail and GOG copies and both of them share this issue.

Is this a confirmed bug and if it is, does anybody know of a fix for it?

My voice lines are missing, too. Strangely, they work when I'm viewing the upcoming campaign mission description, but not once I'm in-game.

I found a solution. After contacting GOG support, they told me to disable all sound devices except the one I was using. I did, and now the voiceover is back! Hopefully this works for you, too.

You mean in Windows?

Sorry for the delay. Yes, in Windows. Hit the start key, type in "sound" and disable all sound devices except the one you want to use.

Thanks.