Merging multiple avi videos (some have audio, some dont)

This is pretty weird so beware. I download a couple seasons of an anime i like (converting avi to dvd) and everything was fine, great picture, great audio. I can fit about 5 episodes on one dvd. For some reason now, some of the episodes do not have any sound. I am using cucusoft AVI to DVD. I merge 5-6 avi files to 1 dvd and 2 out of the 3 episodes have no sound. When i play "those episodes" on my pc they have sound. When played on the stand alone dvd player, they dont. Now if i burn "one" of the episodes with no sound on a disk by itself, it plays fine with sound. This is driving me NUTS. Does anyone have any idea whats going on. If u need more info, please let me know. Thx in advanced guys, i will keep searching the forums for similar issues but please help if u can. Oh yea, i have tried dvd-r and dvd+r.