Most .MID files are intended for an instrument with General MIDI compatibility. You must have the VGM board in your PC88 (included in the PC88MX) in order to have GM compatibility.

You must also turn GM On in your PC88 by going into the Global Menu, scrolling to the General MIDI parameter and turning it On. This reverts back to Off when you turn the unit off, so you must set it each time you power up the PC88.  Some .MID sequences will have a sysex message to turn GM On automatically. Unfortunately, the PC88 does not respond to this message, so you must turn GM On manually.