Why would my music downloads all slow down when I play them?

I have noticed that all my music downloads from UTube are playing slow (like you have your finger on the record for those of you who are of my age group). So Johnny Cash says “Welllllllllll Yooooor Myyyyyyy Frienddddddddd…........”. What would cause this problem? I have not had any virus’s that I am aware of. I am running windows 7on this computer. Any ideas?