I have just bought my first dvd burner, its a HPdvd640.

The software I use to create/burn the DVD's came bundled with it,
ArcSoft Showbiz 2

All seems to be working fine. When I try to burn a file I have
downloaded (usually in .avi format from emule) it works fine for tv
episodes that are around the 350M size - good picture good sound, but
the ones that are around 750M come out with a great picture, but with
no sound.

Am I doing something wrong?

The DVD-R discs I am using are Verbatim ones. I have tried the cheap
ones (£20 for 50 discs), and more expensive ones (£10 for 5 discs)
and the same thing happens on both.

Is there any freeware that would solve the problem? Or is it something
more basic than that?

Thanks in advance.