iPod help? Music vocals not loud, mpeg files?

So here's the deal, I have an iPod Nano, I'm not sure which generation, the one before the square version. Second generation I believe.

So I didn't get these songs from iTunes, but I have them downloaded from somewhere else, paid like 10 cents each. They are in MPEG Audio Layer form. So I synch all the songs that i have, which is only 2 for now, and let my iPod charge a bit.

I try to listen to the songs, but when I do, I can hear the background music and everything clearly, but not the actual vocals. And the whole thing sounds kind of scratchy. I'm not sure why, the headphones are plugged in, all the settigns are at normal, I have no idea what happened.

I also don't use the iPod often. Do you know how I could fix this?

PLEASE HELP ME! I have many more songs that I can't load here because I can't hear them.

:(