Mary,
Do you get the same error message when you try to attach audio file to data file that worked before? It is possible that your new audio file's .wav format is not supported by CLAN. CLAN support only non-compressed 8 or 16 bit and 8, 11, 16, 22, 24, 32, 44 or 48kHz rate .wav files.
If your new .wav file is the same as your previous .wav files and meets format supported by CLAN, then may be CLAN got into a bad state and you should quit CLAN completely and re-start it again. if it still fails try to reboot your computer.