Whenever Yeesah or Esher gave one of their speeches, the voices were mostly inaudible. There would be sudden increases in volume occasionally, allowing me to make a few words, but mostly I couldn't hear what they were saying.
I fiddled with the various in-game audio settings, but it didn't help much.
Then, I found the problem on the "Speakers" tab of the Windows "Advanced Audio Properties" dialog box (get there by clicking Start > Settings > Control Panel > Sounds). The "Speaker Setup" was set at "5.1 Surround Sound Speakers". I changed it to "Stereo Headphones", and all the voices became perfectly clear.