Just tested it in MAME 0.260, it's still glitchy with wrong instruments playing on various midis, with clicks and pops. Guess will have to wait until 0.261 unless someone uploads a recent build.

I've just built the latest MAME from source code and indeed "Yamaha MU-50" is loading and playing its Demo Song in the Emulator: mu50_01.jpg mu50_02.jpg mu50_03.jpg So, yeah, what an achievement - Kudos to MAME ! However, for regular users you will need to wait for the next official MAME release, …