I'm
fairly certain that just renaming the files is not enough. You need to convert the files to WAV format with some sort of audio program.
That aside, I'd make sure the .wav files are the exact same format as the originals in OOTP (16 bit mono, 22 kHz, bit rate 352 kbps, PCM).
Not that I know what all that means, mind you...

Just seems likely that they're more likely to work if they're an exact match.