By design, the sound bar enters Auto standby mode automatically when it is not operated for about 15 minutes and it is not receiving an input signal. This can also happen if the input signal's sound level is low.
This function is designed to cut down the power consumption of the device, and the timer cannot be changed. Your device is thus not malfunctioning. Make sure the sound bar is receiving an input signal - which is strong enough (high enough volume) - to prevent it from entering Standby mode after 15 minutes.
- If you are using a set-top box that has a volume setting, increase the volume on the set-top box (preferably to the maximum setting, or close enough). Then, adjust the volume on the sound bar itself to obtain the desired actual volume.
- If connected to the headphone output of a TV, in the TV's sound settings, set the output to external output. If this cannot be configured, increase the volume of the TV.