A free app for Android, by Alaskavinh.

The RYT – Sounds is your music app on the go. A free app that is very simple and allows you to play music in the background while you are doing something on your device. It also has an uncomplicated interphase since you can use the lockscreen or notifications area to control the player (change music or volume). Additionally, it supports most Android devices and supports mp3, m4a, flac, wma formats. 

This app also has 20 or more pre-set themes which you can choose from depending on your preference. Moreover, this app has low energy usage, so no need to worry about its consumption.