Make sure the music volume on Spark’s top panel is turned up.
Make sure the volume of your audio source (smartphone/tablet) and applications are turned up.
If there is more than one connection, the priority will be AUX > USB > Bluetooth. Only the audio of higher priority will be played. Please make sure to disconnect the audio channels not in use.
Close all background apps on your device to avoid interference.
Turn off Bluetooth on all nearby devices (Bluetooth speakers, Apple Watch, etc) to avoid interference.
Try another iOS/Android device if possible and see if that helps.
