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Question: Question about CD's

I burned several MP3's onto CD-RW discs and have been playing them for quite some time. I took them out, erased them, reformatted them and burned new (MP3)songs onto them -and now they won't play. I tried using CD-R discs, same thing.
When I looked at the instruction manual, I was shocked to read that the CD player is incompatible with MP3 -- since that was the song format on my old CD's.
I don't understand why my newly burned CD's won't play. I then tried using different formats: WAV and WMA. It still won't work. What am I doing wrong, and why did it work before and not now?
PS. I did finalize/close the session on the CD's.