Recommended method:

Connect HDMI cable from Astro HD box to your Samsung TV's HDMI port like normal.
Then, connect another HDMI cable from your TV's HDMI ARC port to your soundbar.

Alternative method:

If you prefer to use digital optical cable, you can also connect optical cable from your TV's digital optical output port (recommended alternative method) OR digital optical port on your Astro box to your soundbar's digital optical input directly.

And then on your Astro remote control, press "home", then Settings>Digital Audio

Change "Stereo (Analogue and Digital)" option to "Dolby Digital 5.1 (Digital only)" by pressing right arrow button on your remote.
Press "ok", and then "Yes, save changes"

You will be able to enjoy Dolby 5.1 audio on Astro now.
Do note that Dolby 5.1 is only available on HD channels, and not available on SD channels.

Can you double check that you're not connecting RCA cable from Astro box to your soundbar.

If Digital Audio Settings on your Astro box is set as "Dolby Digital 5.1 (Digital only)", and you connect RCA cable (a.k.a analogue audio) from Astro box, the audio will be muted whenever you watch HD channels. Dolby 5.1 audio can only work on either HDMI or Optical/Coaxial output on Astro.

Alternatively, do make sure the Sound settings on Samsung TV are set as follows:


I'm using Samsung TV too, and yeah, my TV are able to output Dolby 5.1 audio to my home theatre system via optical cable without issues.

yes, i double checked. but i think i didnt set audio format to dolby digital., instead dts neo. tq, will update soon, kindly working.