Ceres BT speakers have an energy-conservation mode that puts the amplifier on standby and puts the Ceres to sleep. The LED above the volume knob will then turn red after 15 minutes of inactivity. This eliminates the need to turn off the speakers when not in use.
Sending audio to any of the inputs, including via Bluetooth, will wake the speaker from sleep. You will experience delay up to half a minute until audio playback is heard.
As a workaround, you can start playback, and once it starts (so that the connection is awake), stop play and rewind or reset your audio to the beginning to hear the entire selection.