Imported files from another session not working

I edited 3 interviews I have done in 3 separate project files. I imported the files into my final project folder and they copied over easily. I edited them together fine and then went to save the file. I said one or more of the files came from a different sources which I opened subsequently and it did nothing. Then after the files saved the audio i had edited blacked out and did not play the audio like it was on mute when it wasnt. HELP!!!