Windows Audio Converter

I save all my music as wave files, but for putting it on my ipod I convert with Windows Audio Converter. It all sounds all right, but I always get a “Completed With Warnings” message for every track converted. Any idea why? Convert is from 1411 kbps to 320 kbps, if that helps. As I say, they sound fine on ipod but I’m just worried it’ll turn out to be a big problem somewhere down the line.
Thanks for any help!