For some reasons Sony Z series team messed with walkman lockscreen widget. There was no controls available @ lockscreen whilst you quickly want to pause or change music tracks.
Playstore has a good alternative intact better than Inbuilt widget called “Music Widget for Walkman”.
I’m loving it ever since I installed (1.5 days 😉
This widget works on Sony Xperia Android phones. It’s worth to give it a try!!