Sorry for not answering earlier.
I have exactly the same issue with this voicepack as described (used one from this link http://www.mediafire.com/download/7j...PackBETA10.zip) and wherever I put it (main folder or voicepacks folder) result is the same - it's loaded but not playing at all (one strange thing also is that, when I press "preview" in any of the achievements there is no soind and RTST is unresponsive (frozen) for a couple seconds and I can't click nothing to exit configuration window).
SpaceMarinesMKII works great, so it is not a problem of RTST or my wrong loading procedure, but rather this voicepack