Most .MID files are intended for an instrument with General MIDI compatibility. The Mark 10 is GM compatible but it must be turned On in order to hear the GM sounds.

Many .MID sequences will have a sysex message to turn GM On automatically. But if the sequence does not contain this message, you need to turn it on manually. To do this, hold the MIDI button and strike the lowest black note on the keyboard, as shown on page 45 of your manual.

You can tell when GM is turned On - you will see a dot after the last character in the display.

Once GM is turned On, the Auto Accompaniment features will not work correctly, since they do not use the GM sounds. Also, any sequences recorded on the Mark 10 itself will no longer play the correct sounds. Therefore, you should only turn GM On when playing sequences intended for a GM instrument.