How to make a fake auditory signal?
Devices that bypass the hair cells in the inner ear and directly stimulate the auditory nerve are called cochlear implants. Cochlear implants are used to treat deafness caused by the loss of hair cells in the cochlea. The hair cells are the sensory cells that convert sound vibrations into electric neural signals (Purves et al., 2001). With state-of-the-art devices, the better performing implant wearers are able to talk over the phone(!) An exclamation mark is in place, because it means they can understand the spoken word without the need for lip reading or sign language. Hence, cochlear implants are the answer to your question, but they definitely are not designed to generate fake signals. Instead, they are used as an effective treatment for deafness and are capable of transmitting meaningful speech information.

Cochlear prosthetics consist of an array of typically 20-24 electrodes that are inserted in the scala tympani of the cochlea. The inner ear is tonotopically organized, which means that high frequencies are coded in the base of the cochlea, while low frequencies are encoded in the tip. Hence, each electrode stimulates a separate frequency band (Fig. 1). By using a microphone and sending the acoustic signal through a filter bank, a number of auditory frequency bands can be obtained equal to the number of electrodes in the implant. After this speech processing step, the acoustic frequency bands are converted into trains of biphasic pulses. These pulse trains are sent to the electrodes in the cochlea, which then activate the auditory nerve directly, effectively replacing the degenerated hair cells in the inner ear (Stronks, 2010). 


Fig. 1. Cochlear implant. Source: Mayo Clinic. 

References
- Purves et al., Neuroscience (2001) 2nd ed.
- Stronks (2010). PhD thesis, Utrecht University