There is Two Steps in this project.
1. A simple C# app (web, win or console doesn't matters) will make a sound signal. I will enter a 6 digit text code (nums and chars, for example 6XNC03) and it will code this in a 44.100 sample rate 16 bit sound code, a duration time, export path and name and it will generate a wave file to export. Then I will convert it to mp3 and put this Wav file to a portable Mp3 player. In the wave file it must have a serial data transfer properties. A start bit, stop bit and a error correction code. It must have also tree different sound frequency. 400,600,800 khz. Two for data, one for make the speaker always have a background noise.
2. I will play this files like this with MP3 players in a market's sections and an embed pc on the staff's stroller will listen this sounds. It is very similar like RFID. When the strollers microphone detects the sound it will do somethings... (count, Start a web service, etc.)
The most important thing in this project is, creating a wave file is simple but listening is complicated because there will be similar sounds or noises at the same frequency. But like Wifi signals (they are also mixed) the app in the stroller will detect and decode it and get the ID from the sound. And if it is a code that we created, it will do the job.
It looks like very complex but someone who knows serial communication and signal processing can do this very easy.
The next step will make an Android version for stroller... But first is first. :)