try Frinky Music Controller ( no root required )
I always wanted to have an app which controls music with volume keys but i could not find any which does it smoothly. So I developed my own

. My app Frinky Music Controller on GOOGLE PLAY controls music with volume keys and smartly controls music volume. It supports more 200 music players including poweramp, musixmatch, doubletwist, la musique, rocket player, blackplayer, pi music player, and Spotify. It works on both rooted and non-rooted phones.
Some features of the app are :
Next Song -- press volume UP key single time to play next song.
Previous Song -- use volume DOWN key to play previous song.
Control Volume -- when we want to control music volume , most of the time we press volume key more than once (next time when you adjust volume , feel it) . So the app uses this tendency of users as a function to control music volume i.e press volume key more than once to adjust volume.
Stop song -- Oh! you play music in your class in absence of professor here comes the professor .... turn off the music quickly by UP+UP+DOWN+DOWN key action (it takes less time than muting the volume and unlocking the screen

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Sleep Timer : Single sleep timer for all your music apps.
Manually control app's service : well to understand this feature you will have to download the app.
It consumes very less amount of ram.
I am new here so can not post link.