How do I pause a song thats playing while my phone is locked (Samsung ace 2) I am looking for a way to pause a song without unlocking it. Take the iPhone for example; just by pressing the home button twice, it pauses the song. It's annoying having to unlock my phone every time that I need to pause a song.
Here are some apps to suite your needs:  

Tactile Player – Control Android Music Player Using Hard Buttons

"Tactile Player is my favorite tool which gets the job done and I must say, this one is a must for any music addict who wants to change songs using Android’s hard buttons or play/pause the current playing song. When you are listening to music and don’t want to unlock Android’s lock screen, simply enter the following commands

Press the volume up button two times to play the next track.
Press the volume down button two times to play the previous track.
Press the power button three times to pause or play the currently playing soundtrack.
It works and the best part is that there are no settings and absolutely no configuration required on your music player app. The only catch is that the Tactile player app must run in the background or else the default music player app won’t be able to receive your commands via the volume and power buttons."

Simply Lock Screen – Replace Android’s Default Lock Screen And Add Multimedia controls

"Simply Lock Screen is a wonderful lock screen replacement app for Android which comes packed with basic multimedia features. You can show clock, date, time and media buttons on the lock screen itself, thereby allowing you to control the order of songs that is being played by Android’s default media player app.

Simply Lock Screen adds three buttons on it’s lock screen, so you can play a previous or next track or pause the current one by tapping the respective buttons on the lock screen itself. This one is a good choice if you are looking for a better lock screen replacement with added features."