Assume a voice signal is being transmitted over a particular FM frequency, e.g., at 87.5MHz.
The user is provided with an android tab on which the FM radio (app available in all android devices) is tuned to 87.5 MHz. The user is listening to the audio using headphones.
Following modifications need to be done in the Android tab
1) The listener should not be able to see or find out that the voice is being transmitted using FM at a particular frequency. This means you need to completely hide the FM Radio application while it is always running in the background
2) You should be able to change the frequency of transmission on FM (but, again, user should be totally unaware of the usage of FM)
3) Develop an application that can launch FM radio app but at the same time also keep the FM radio application hidden so that the listener is not able to find out where the voice is coming from (while it is always running in the background)