Choose default music player to play / pause music on hands-free button press
You could check the playstore for headset control apps. Having a Philips headset, Philips Headset works perfectly for me -- and lets me define which player app to launch as soon as the headset is connected, plus gives me a bunch of control options for the headset's single button (e.c. click for play/pause or take call, long-press to increase volume, click twice to skip a title or reject a call, and more).

There are many more similar apps available. Quite famous examples include e.g. JAYS Headset Control (which did not work that well with my Philips headset -- but that may be caused by the headset itself), and Headset Button Controller. Simply search the playstore for "headset control" for more options.