Samsung HW-E450: no sound using HDMI (arc) connection

Please help !!
I just bought a Samsung TV UN55ES7100 and soundbar HW-E450.
I tested successfully with optical audio cable. However I have trouble with HDMI.
Here is my setting:
- Cable box connected to TV's HDMI 1
- soundbar HDMI out to TV's HDMI 2 (ARC)
- HDMI-CEC (anynet+) are enabled on both TV and soundbar
soundbar's source selected is 'HDMI' (not 'D.IN')

With above setting, I can see volume bar when I use TV remote volume control, but I cannot get get any sound from soundbar.

I chated with Samsung support online, they insisted I need a optical audio cable in additonal to the hdmi(arc). However from this forum, many postings suggested this is not necessary. Am I wrong?

Any help greated is appreciatd.

Ken

Thanks Doctego,
I finally made it work. What I did was that I temporarily connected the cable box to the soundbar HDMI in, and soundbar HDMI out to TV HDMI 3. This way I was able to have both picture and sound, just could not adjust sound volume using TV's remote. Afterwords I put everything back to the original setting as described in my previous message. Somehow it mystically worked.

Ken