I'm assuming you are running some version of windows.
But before I say what you could use, I have to give THE STATEMENT.
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When converting from one music compression type to another (examples of music compression are wma, mp3, ogg, etc) you will experience a loss in the quality of the music. A converter MUST change the compressed file that has already lost some of its original quality into an uncompressed wav file. Then converter then will REcompress the file into a new format. This double compressing strips away a large amount of the original quality. In most cases, you are better off finding the original and ripping straight to the type you want. If that is not possible, then you can convert file types and hope you do not mind the difference in quality.
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that being said, I have used DBpoweramp before and have been pleased with the results.
http://www.dbpoweramp.com/dmc.htm