I really need help with syncing to my iPod- songs full of static, AAC versions just don't sync?

I downloaded some songs from iTunes a couple of days ago and then synced them to my iPod (which is manually synced due to my songs being split between computer accounts- long story ^^). The songs themselves synced, but they're full of static or just silence. One in particular you can hardly hear- once the song starts playing, it just becomes quieter and quieter, interspersed with plenty of static. The other songs on my iPod are absolutely fine, it doesn't appear to be a problem with the iPod or the computer. I've tried making AAC format versions of these songs (which are absolutely fine on iTunes, just suddenly damaged on my iPod) but they won't sync- I drag them over to my iPod's icon and nothing happens. It worked before with a song that was just silent. I really need some help, I am so frustrated! Thanks in advance for any answers.