The animation is really nice, i also like the colours.The only problem is the sound from the headphones, you should have leveled that.

You should learn how to edit sounds, put in an effect or edit the fitness gram (or whatever that running shit was) so its sounds like its muffled and actually sounds like its coming from the headphones.
Give your cartoons more substance. This cartoon has nothing really going on and the punchline wasn't that good. Maybe add a couple a scenes where the other two character find out what he is listening to and react to it? I dont know just give something more that what you have already
also mouths dont move like that, give the mouth movment more frames so they dont go that fast