OP, all I cared about was the speakers for the size I purchased. And on that note, the speakers on the 12.9" sound fantastic.
If they are better on the 10.5", politely asking, is this a deal-breaker for you? Are you an audio geared person who requires the absolute best in speakers or something else?
If the 10.5" sound better than the 12.9", that's great. As an owner of a lovely 12.9", I am not worrying about it and enjoying the awesome sound from the model I chose.