How to adjust the voice volume level in Google Maps navigation (Android)
Open the Google Maps app, and tap on the menu button that you can see in the upper left corner. Then, scroll down to the Settings tab, and tap on it. After that, you should see a Navigation settings tab - again, tap on it. Inside the Navigation settings sub-menu, there's a Voice level tab that will, of course, allow you to change the voice volume. Unfortunately, there's no slider to let you adjust the volume to your heart's will. But there are three voice level options that you can select: Normal (this is the default), Louder, and Softer. The differences between the three levels are considerable, so this is, undoubtedly, better than not having options at all.