What are you using to convert? A few people have had this problem after converting with Videora. I've used these settings & had no volume probs:
Bitrate 128kbps
Channels Stereo
Sample Rate Input(default)
Volume 100%

You might find you have more success with 3GP converter. There's a lot of info below, lots of it about adding video to iPod
Transferring music to iPod & iTunes

If you use the forum search to look for 3GP converter, you'll find a few threads! Personally, I think 3GP's much easier to use