Quoted from Mokthemagicman:

I also put part of this post on the MMr Mods page:
This should be a "must do" change on EVERYBODY'S MMr!
Everyone has been complaining about how muffeled the voices are, get drowned out, and how low they sound when compared to a classic MM machine. When I received my MMr I also noticed that the voices were getting drowned out from the music. I read about people swapping out the cheap "Made in China" (written on back) speakers. They lack a real tweater and run at a 86db sensitivity. I took advice from fellow pinsider Nosro and upgraded the speakers with Kicker 40CS54 that has 90db sensitivity. He said that the old cheap speaker was only a "mid-range driver that doesn't even have a whizzer (which is the inexpensive way to get some treble compared to a dedicated tweeter). This is why voices sound muffled."
Interesting right? So I took the plunge and decied to try em out.

First I must say that the speakers install perfectly into this machine. It almost feels like they were "supposed" to be the correct speakers installed into an MMr. They fit into the circular frame lip like a glove. A few solder here and there to the wires and boom. New speakers and WOW what a HUGE difference! Let me see if I can properly explain the quality increase in sound that will make sense in word form, without you actually hearing it.
First off when I turned on the machine and started a game I was instantly blown away with how the King's voice was much, MUCH louder, clearer, and cleaner. It sounded very separated from the music and not "mushed together" in a muddy way that the old cheap stock speakers sounded. So really the problem wasn't the software not having an option to turn down the music. It was always an issue with the speakers not being able to reach the proper pitch and sound sensitivity to make it sound they way it should. This is really too bad. So an update to the software for adjusting the sound isn't the answer. Upgrading the speakers is! I guess the added 3db makes a huge difference.
As I played I also could hear a distinct separation of music instruments such as the snare drums and horns. They definitely sounded sharper and higher resolution. The bass was greatly improved and increased too, noticably when you lose a ball and the scores get added up. Such a distinct difference! Another cool thing that really stood out was a certain sound effect. When you chop off the Dragon's Head the sound of the slash "Shhhhink" sound was at a higher and clearer pitch. It was so much louder that it actually made me look up at the DMD screen and take notice. Not so loud that it hurt my ears or anything. It just was VERY noticeable, when before it just kinda got lost and muffeled when squished with the music like all the other background sounds did (or used to before the upgrade).
Overall, the swap out with the Kicker speakers turned out awesome and I am very happy with it. As I said, this really is a MUST for any MMr owner. It's a shame that the creators didn't put higher quality speakers into this machine because it really needs and deserves them. I feel bad for any MMr owner that has NOT made this update to their machine yet and I think they are missing out on such a great improvement to their machine. If you are not thrilled with the quality sound, music and get frustrated that you can't hear the call outs or the funny lines from all the various characters in the game then do yourself a favor and make this upgrade a priority to your MMr machine. You will love it! At least I do.
http://amzn.to/2bYDTHR
Here is a link to my $59 Kicker upgrade if you want em: