You can't pass HD audio over ARC.
ARC can only handle (in theory) the same Audio formats and bitrates as Optical.
I say in Theory because there is a brand of TV in North America that reports it can send DD+ via ARC, although there seems to be some bit rate reduction so it is a grey area.
Connect your TV's ARC connection to the HT ARC connection, If you then plug something into your TV it will pass 5.1 sound over ARC to the HT at up to Dolby Digital 640kbps / DTS 1500kbps.
If you require HD audio from your box, you will need to connect any device capable of HD audio via HDMI directly to your HT.
select the HDMI input on your HT, and on the TV select the HDMI input you used, in this scenario ARC will not be used even though it is still connected, Your HT box should extract the audio to it's speakers and pass the video to your TV.