Conversion avi to ipod mp4, works without audio

I've tryed to convert 2 avi video to iPod mp4 format. So pytube conveted them correctly. Trying to open them with the pc all works great, video and audio ok. I tranfer them to my iPod Video 30gb (iPod 5.5 generation) I try to play them and video is ok, but there isn't audio.
Other video converted with other software work perferctly. I don't know if there's a problem of codec version...

I hope I can be helpful