I'll second what yancy stated regarding the highs and MMR's midrange speakers. MMR sounds flat due to the lack of a tweeter like the original games had.
I took a different approach than him The original MM had one 5-1/4" speaker on the left and a tweeter on the right. I set my game up like an original, meaning I removed the 5-1/4" speaker on the right side and installed a tweeter in its place. The main reason I did this is because I had a tweeter on hand and could easily improve my sound without having to order and wait on new parts. You also have to install a capacitor on the tweeter so filter low frequencies, I don't remember what value but I used the same as what the original MM used.
Doing this significantly improved the audio. Whatever approach you take, adding a tweeter will make a noticeable difference.