How do I access everything on a DVD-R?

I have a DVD which I use as a back-up at file level. I wanted to copy an audio file from it to another DVD but it copied it to the same DVD. Now I can only access that audio file on pc. If I put it in a DVD player it'll play the video I have on it. It's not re-writable so I know everything is there somewhere. I have audio, image, video and text files on it... somwhere!