If you think songs on your iPod shuffle could sound better, it could be a result of transcoding. Transcoding will convert higher bitrate (160 or 192 kbps) MP3 files to 128 kbps AAC files if the preference is set this way.
To prevent this, go into the iPod preferences and turn off the "Convert higher bitrate songs to 128 kbps AAC for this iPod" option. Another way to ensure good quality is to rip the songs from the original CD directly to 128 kbps AAC files.
This option was designed primarily for larger AIFF or Apple Lossless Encoder files.
A note about copyright
This software may be used to reproduce materials. It is licensed to you only for reproduction of non-copyrighted materials, materials in which you own the copyright, or materials you are authorized or legally permitted to reproduce. If you are uncertain about your right to copy any material, you should contact your legal advisor.