As Tyler1989 said...when you re-encode an MP3 file, or a WMA file for that matter, you'll only get the same quality, or lower depending if you re-encode at the same bitrate, or lower...
Just in case, re-encoding say, a 128Kbps MP3 file to a 320Kbps mp3 file will not increase quality, because the original source has a lower bitrate than the new source. So if you mean, increase the 'quality' as in increase the 'bitrate' of the audio file, then yes it's possible to re-encode to a higher bitrate, but quality will not increase...it will stay the same.
The only way to improve the quality of audio files that have already been encoded into MP3, WMA or any audio format for that matter, is by 'cleaning up' the audio yourself, like using the noise removal feature in Audacity or the Filter Toolbox in Nero WaveEditor...
...Alternatively, to gain better audio quality, then you'd need to obtain the original audio source and re-encode to a bitrate which gives you adequate quality...
...For example, ripping (encoding) tracks from an audio CD to MP3 files at 320Kbps, will be better quality than ripping the tracks on the CD to MP3 file at 128Kbps.
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