You will need to turn them on in the notification center on your phone.
Go to your device’s Settings app, access the Notifications settings, and select the Online Sales Pro app there. Make sure that the options for Allow Notifications, and Sounds or Alerts are all enabled there (as well as adjusting any other available notification settings for the app to your preference).
Next, go back to your device’s Settings app, access the Sounds and Haptics / Ringer / Volume settings. Ensure the Ringer/Alerts sound for your device is turned on there, and that the device volume is high enough to easily hear.