It has been a struggle to get Sony Xperia ZR’s Walkman App to recognize music files on the external SD Card. Unfortunately, sometime ago, it detected the Micro SD card and built the play lists, but lately, it does not do that as well. After installing Android 4.3 on the Sony Xperia ZR too, the problem still remains.
Due to this struggle to get it working, I decided to move away from the Sony Xperia Walkman App, even though it offered the best audio experience within a headset and installed Clean Music Player (a simple music player) and to create a range of equalizer effects, Bass Booster from the Google Play Store. Above all, bass booster also boost the sounds from your internal phone speaker. However, exercise caution when setting up the bass for the internal speaker.
Below are the links to the apps to help overcome the loss of the Walkman App.
Clean Music Player at https://play.google.com/store/apps/details?id=com.myskyspark.music
Bass Booster at https://play.google.com/store/apps/details?id=com.desaxedstudios.bassbooster
Killer combo to get good music into your ears.