Welcome to the Gnutella Forums Jestudi.
There may be a host of different reasons why you are hearing what you're hearing.
I can think of a few possible reasons.
May be the encoding software you're using.
Might be Burning at to high a rate. The slowest possible is the Best.
The Media you're Burning on might not be the best. Try to find Domestic Disc, and Never use Store Brand Disc's.
Try Maxell Disc's.
The one that sticks out the most is Bit Rates. The Higher the Bit Rate the Better the Quality. If your downloads are below a 196 bitrate, try finding either 256 or 320 and see if you can tell a difference.
Bitrates? 128, 160, 192, 224, 256, 320 are almost always
CBR (Constant bit rate), those that are between are
VBR (Variable Bit Rate.) More about bit rates here
crazy bitrates? I prefer
VBR myself as they tend to be better balanced in their sound.
Hope this Helps.