Mp3 slowing down

Hi, I’m making a simple mp3 player with green_shoes and rubygame,
everything works alright but after about 20…30 Up and Down keystrokes
to change the tune it starts to go slow and I’ve tried various things
with no success.

Please could someone take a look at the file and tell me why it is
slowing down, it would be most appreciated.

NOTE: when executing the file you have to add your own images to click