It is either your source, the RW or your drive.
I like audio and on a couple of drives I have some RW sound poorer than on others using the same files as test.
In older days when I used to use Audiograbber if yor CPU was not fast enough and say you converted to mp3 on the fly without wav, if at anytime the cpu buffer went into RED you got very poor conversion( very high treble and hashy sound)