i've recordings from previous versions too but no problems in the file structure. could you check if anything else is causing the problem?

P.S. if your folder is not hidden, i.e. without the "." in front of it, any file you play from within the folder will play in the music player but not in the call recorder app. it's vice-versa if the folder is hidden. just posting it in case someone else has trouble figuring this out.