My favorite daily media player for audio and some video is OPlayer which is terrific for playing downloaded MP3 files or podcast type audio files with very low CPU and memory usage.
Once you have OPlayer set up correctly it can be so enjoyable to use with one-hand. OPlayer has easy sliding gestures for seeking forward and back along with volume and brightness control and comes with playlists and more.
Tips in Settings:
Turn off "Simple Player Screen"
Settings / Theme: Turn On "Display File Name in 2 Lines"
Turn On "One Hand" Controller by using the (three dots ... menu) in upper right corner to access six different player features including a Sleep Timer and more.
Lastly I hope the developer continues support for iOS 8 for older 4S devices which are used as dedicated media and audio players :-)
zeppydog about OPlayer - video player, v3.2