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.
If you use a headset when listening to your music, and that headset has a button (usually for accepting incoming calls), you might want to take a look at the many headset controller apps at Google Play. They allow you to play/pause with a short single press, skip forward/backward with double/triple click, adjust volume with holding the button, and much more.
As not every app/headset/device combination works 100% satisfying, don't give up on the first try if it doesn't work out "ideally" -- but try the next one. Took me 3 tries to find the perfect match for my device/headset :)
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.
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."