since all my audio sources were download files which have 128kbps in average, if I want to make mp3 cds with them and improve their quality, can I convert them to wma files and then back to mp3?
How in earth you think that MP3->WMA->MP3 conversion would improve quality? If you trascode from LOSSY compression format to another you will SIGNIFICANTLY lose quality in every compression round. You are taking your MP3s from the wrong places. Play around with the DC++ and you'll see a lot of 192kbps stuff, which is not great but much better.