NERO sound problems

Can anyone please advise me...

I've currently got NERO on my pc...when burning MPEG video clips from my hard drive onto DVD there's no problem with sound or vision BUT when I try and burn "windows media audio/video files" (from my hard drive) the picture is fine but the audio sounds all speeded up..it does this in preview not just on the DVD itself (everything's fine when I just play the clips on my pc)

Do I need to convert my "windows media audio/video files" to MPEG?...if so,how easy/time consuming is that and how do I start going about it?

many thanks in advance..LB.