The original headphone that comes with ipod does not really has good sound quality...but if you change to better headphones like sennheiser, crossroads mylar, koss etc...the sound quality improves so much...it is nice...

Actually sound quality depends on few factors:
1) Your MP3 player itself - the sound "processing center"
2) The quality of your earphone - this is like your speaker
3) The bit rate of your MP3 songs - If you are installing a song with only 128 bit rate, sure it will sounds bad as compared to a song with 320Bit.