Unable to control external speakers connected using 3.5 jack

I have a samsung LA32C530F1RMXL model. I have connected a 2.1 speaker system using the 3.5mm jack. You mentioned that if the 3.5mm jack is used then the remote could be used to control the volume.
This does not work for me. I am unable to use the remote to control the volume and must rely on the volume control of the 2.1 speaker.

Is there any specific settings to be done?