Copyrighted or wrong format...
Working as a store manager for Verizon Wireless, over time I have seen difficulty using song converting software to allow you to play it on your phone (and or PDA device), ESPECIALLY with I-Tunes. Some music is copyrighted in such a way that even if your conversion software 'says' that it converted for you, after you sinc your handset with your computer, it will still refuse to play it. I have even seen cases where it will show the song playing on the song timer, but no sound. Your phone will say exactly what format the song and or songs must be in in-order for it to play them. So make 100% sure that is the format that your converting them to before you go ahead and do your whole Library thinking that it will play fine on your handset.