A free program for Android, by Indra Aziz.

This app is a practical and simple way to do a quick and easy warm-up of the voice. It helps you to be prepared for any kind of speaking or singing situation, such as in front of a class, before an audition, or during a competition. 

It is a free app with ads. You can remove the ads for a small fee. The instructions and audio guide are very clear and easy to follow.

This app includes two routines: one for warm-up and the other for breathing. It has various exercises, and you can use them to practice and improve your voice.