When next flying your aeroplane use some decent headsets to protect your ears
, sorry, couldn't resist.
If you did not alter the files and just burned them on a cd, there should not be any change detectable, unless you leave the cd in bright sunshine on your dashboard and it transformes in some cheeseburger style topping....
When you download MP3s from the net, the quality of the each file is only as good as the ripper produced the file, often there are some individuals who "play" too much with all the available settings which in turn results in some "strange" effects. I am very sure that the file you used to burn sounds exactly as the one on your cd, unless your cd-writer got problems but, you should have noticed that straight away while listening to it the first time.