I have found lots of examples how to get default ringtone. Something like that:
Uri alert = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_RINGTONE);
This is the extension of dgabor's answer.
Just add defaultRingtone.play(); to play the ringtone.
Uri defaultRintoneUri = RingtoneManager.getActualDefaultRingtoneUri(getApplicationContext(), RingtoneManager.TYPE_RINGTONE);
Ringtone defaultRingtone = RingtoneManager.getRingtone(getApplicationContext(), defaultRintoneUri);
defaultRingtone.play();