Sounds like the metronome is on. Version 15 of Music Maker? Just hit "P" on your keyboard. This will take you to the audio/MIDI settings tab. Find the "Activate metronome" checkbox, untick it and press OK. Done!
I hope this helps.
P.S.: The metronome checkbox in the MIDI Editor window is also linked to that. It turns the same metronome on and off.