I found this very interesting. It marks the very difference I've been trying to nail down in why I prefer classic game soundtracks to new game soundtracks: classic games had no voice acting. The article points out that games with voice acting cause the brain to focus on the words instead of the music and there's "not enough bandwidth" for our brains to process both voice acting and an interesting soundtrack. The soundtrack takes a backseat to the speech because that's naturally what our brains focus on. This is also why Hollywood movies and video games with a cinematic flow tone down the music for when characters are talking and leave the lead melodies to areas without talking and intro/credits sequences.
This is exactly why I prefer the soundtracks of old and I didn't even realize it: speechless games. Even the soundtracks from LA's games experienced without speech are far more engaging (the music, not necessarily the game). It's a very interesting difference. In the article the author links to two YouTube videos from FFVII, both of a sequence with text and engaging music (yeah, that's most of the game, but you know). One version was the original game presentation and the other with voice acting. The point was to show just how much more effective the music is when there's nothing else to listen to, and it's remarkably true! Maybe you guys understood this difference already, but I've never thought of it that way before. Perhaps that's why I always preferred KQ2+ without voices and why I never found TSL's or Telltale's game soundtracks particularly memorable or striking (though done extremely well). Though I really enjoyed Puzzle Agent's themes....particularly the puzzle themes, because there's no dialogue!
You know, in light of this I really think it'd be great to play a game solely based on gameplay with a rich soundtrack and no dialogue at all. Speech nor text. Just music telling the story. That would be an interesting experience and a fun exercise to score as well. I'll have to do that before I die sometime...