I think the problem is having the game on a different drive letter than C: I've gotten more than a few emails from people with the game installed to a drive other than C: and have problems, so you're not alone
All you need to do is to re-assign the speech file yourself. To do so:
1) Open up the .mis file in the level editor
2) Near the top of the screen, you'll see things like Select, Tile, Entity, Entity Edit, etc. Click on Level
3) Near the bottom-left of the screen, you'll see some buttons. One of which is 'Speech File...'. Click on it.
4) Find the speech file (rebellion_speech.txt) and double-click on it. DO NOT SELECT IT AND CLICK 'OPEN'. It won't work. Don't ask me why
5) Save the map (File -> Save).
6) Start up the map as you normally would (using Custom Campaign) and play it.
7) Post in the Map Reviews forum after you're done :mrgreen:
That's all there is to it!