by Fizbin » April 30th, 2013, 6:28 am
It depends on how you're making the subliminal. If you are simply adjusting levels and putting the file under another, audible, track, it is pretty difficult to extract the original message.
If you're using an ultrasonic frequency shift, that is reversible, though the fidelity depends a lot on the sound going in.
This piece of Nyquist script will decode most common ultrasonic subs:
(setq carrier {your carrier here})
(lowpass8 (mult 2 s (hzosc carrier)) 8000)
replace {your carrier here} with the frequency base used in the original conversion. You may need to experiment to zero in on it, and it could be different for each file.