Can I use side buttons of my phone to turn (by holding) next/prev track without root? I have xperia M. It has normal walkman player. But choosing music is not comfortable, it needs taking out my phone from a pocket. How can I custom side volume buttons to turn next/prev. track by holding it a while? (How it was in old walkman telephones)

Thank you in advance.
There are several possible approaches to this.

If you're using a headset (which I assume you are), there are several apps for headset control allowing you to use your headset button for that (e.g. one click = pause/play, double-click = next, etc.).

On the other hand I've seen C Locker is capable of exactly what you're asking for: using the volume rockers for that while the screen is locked.