I have searched on the net for a hearing aid circuit diagram becase I wanted to build a hearing aid device or as you wish a spy device. It amplify the voices and sounds.
But there is an another way to make a hearing aid if you have and android
phone. The phone has a microphone, a headset and an
amlifier is in your phone. There are some softwares on Google play store (most of them are free):
SoundAMP with Rec
Hearing Aid – Cochlear