The name of the application was changed from SongPal to Music Center in May 2017.

Music Center (SongPal) Help:
http://songpal.sony.net/en_ww/help/

Refer to the following questions and answers:

Q: Where can I get Music Center?
A: You can download the Music Center app for free from the Google Play store on Android devices or from the App Store on Apple iOS devices.

iOS：https://itunes.apple.com/us/app/songpal-bluetooth-wi-fi-audio/id724406878?mt=8
Android：https://play.google.com/store/apps/details?id=com.sony.songpal&hl=en

Q: Can the Music Center be used with a USB DAC player?
A: No, it cannot. When using a USB DAC, use a music player application that supports USB output.

Q: The source screen on Music Center does not open.
A: Confirm whether a Bluetooth connection has been formed with a connected device.
If the Music Center  source screen cannot be accessed from the Speaker & Group screen, despite being connected by Bluetooth, temporarily close Music Center and then re-open it.

NOTE: For Music Center (SongPal) group playback, refer to the FAQ below.
Frequently asked questions about group playback of Music Center (SongPal)