Wouldn't you like to put the Wi Fi enabled smartphones that you are not using anymore to some use ?
I decided to build the sound lookout app one fine day, when I missed hearing the door bell while sleeping in the bedroom that is seven steps up from the living room in a duplex flat. I thought of putting one of my un-used Wi Fi enabled smartphones to use and enable it to talk to another one via Wi Fi and make it play music once it hears the door bell. So one would be Listener which is listening to the door bell and another would be Alerter , which would lie beside me on the bed so that I am awakened by it when it plays the music. That is why I think it is a Me-2-Me app!
The sound lookout app will have to be installed on two Wi Fi enabled smart phones, one will act as Listener and another as Alerter. In the settings screen for the Listener, the IP address of the Alerter will have to be set. Most home Wi Fi routers have the capability to set static IP address corresponding to a specific Wi Fi MAC address. The tech savvy folks can update their Wi Fi router settings to assign a permanent IP address to their Alerter smartphone.
Couple of links pointing to how to set permanent DHCP IP Address for certain Wi Fi enabled devices.
TP Link Wireless router
http://www.tp-link.us/faq-182.html
Huawei Wireless Router (Used by Airtel 4G in India)
https://www.emt.ee/files/eshop/files/Huawei%20E5172%20kasutusjuhend%20ENG.pdf
The sound level that has to be detected can be changed as per circumstances. You can increase or decrease the default value seen in the settings screen under "How loud a sound to be detected" as per your desire. Suppose with the default value, in the Listener mode, the sound is not detected after you click "start listening", then you can lower the value, restart the app and check if the sound is detected after that. You can follow this process accordingly until you are happy with it.
Uses of the app
1. To know a door-bell has rung while you are away in your bedroom sleeping or listening to music with headphones on.
2.
To know that a baby sleeping in further away room, has started crying which otherwise would not be possible to know. This typically would be the case when the baby is sleeping in a room upstairs in a duplex flat while you are in the living room watching TV.
3. To keep a little kid entertained by showing that a music starts playing on another phone once the kid claps near a phone. Be ready for endless requests of "again" from the kid !