Mosquito Sound is a free app developed by Just4Fun Playground for Android 9.0. It allows you to play sounds at frequencies between 17.4 kHz and 20kHz, and even up to 22kHz, which are called Ultrasounds. With this app, you can test your audio devices, check what sound frequencies you can hear, and train your dog with high-frequency sounds.

One of the main features of Mosquito Sound is testing your audio devices. You can use this app to check if your audio devices, such as headphones, speakers, and home theater, can play sounds at certain frequencies. Another feature of this app is checking what sound frequencies you can hear. It is common for people over 50 years old to lose their ability to hear high frequencies, and this app can help you determine your hearing range.

Lastly, you can use Mosquito Sound as a dog whistle to train your dog with high frequency sounds that are audible to dogs but inaudible to most humans. However, please note that some built-in phone speakers may not be able to produce all sound frequencies in the range of 9kHz to 22kHz.

Overall, Mosquito Sound is a useful app for testing your audio devices, checking your hearing range, and training your dog. Give it a try and see how it works for you!