The MOSFET's, for one, are not a good choice. Their on-resistance is too high (0.55 ohms). With 6.5A of current flowing their power dissipation could be as high as 6.5^2 * 0.55 = 23.2W. That will fry the MOSFET in short order.
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The Arduino Drum Machine: 14-track MIDI drum machine sequencer / groove-box