The next version MEWER is online. The difficulty level has been calibrated, and I've added a third variant of MEWER. This new variant includes an audible metronome! However, the metronome is quite unreliable. If it works on your computer, great; if not, don't say I didn't warn you. Try it here: MEWER 0.9.
I still think MEWER is now feature-complete for 1.0 -- I'm not planning on adding any more features; merely fixing any bugs people find.
 My preferred calibration method was to have four glasses of wine and then play the game on "easy". I figured that I'd keep on reducing the difficulty level until I could finish the game while drunk; that would probably be an appropriate level for students with one year of musical training.
But then I remembered that I don't drink alcohol, so I had to abandon my "drunken rhythm game" test. Instead, I started watching some bad anime, and played the game with my attention split between the anime and the game. I finished "easy" with no difficulty. Calibration for the "hard" difficulty level was performed by playing the game without any distractions.
This isn't an ideal scientific calibration, of course -- I should really be doing some kind of double-blind study on dozens of music students of various ability levels -- but I can't do that kind of experiment until MEWER is much more widely used.