I downloaded an MP4 video from Youtube using an addon to firefox. Now, I want to make it into a dvd for resume experience(3 parts, 6 parts). I need to merge the parts together into one file before using DeVeDe. With avidemux, I get this when I load the first part:

H.264 detected: If the file is using B-frames as reference it can lead to a crash or stuttering.
Avidemux can use another mode which is safe but YOU WILL LOSE FRAME ACCURACY.
Do you want to use that mode?

Safe mode or cancel corrupts the sound. Is there a way to fix it through avidemux? I'v tried to convert to avi format to fix the problem. It didn't work. Also, I’v searched the internet for more info.