You have backed the files up as "audio" files so it has converted them into the format used on audio CDs. They are not mp3 files any more.
You should have just backed them up as "data" files and it would have put them on the CD as MP3 files.
You need to "rip" the tracks off the CD and convert them to MP3 files again.
I think Windows Media Player can rip tracks off a CD, if not there are loads of programs available on the internet that will rip tracks off a CD and make them MP3 format.
Just search for "CD ripper"