Are the volume buttons on your device broken? Try Volume Slider For Android, the volume button replacer. Control up to four different volume streams on the same screen by simply sliding your finger up or down the edges of the screen. It's that easy, and bonus,

Volume Slider For Android now controls brightness as well.

Note :
If the app isn't working for you especially on Redmi devices, please go into Settings or wherever you grant your permissions, and make sure Volume Slider For Android is allowed to DRAW OVER APPS > DISPLAY POP-UP WINDOW > PERFORM SYSTEM_ALERT_WINDOW > MODIFY SYSTEM SETTINGS.