Yea, I would expect the volume on the ARC to be fixed, just like if you were using an optical out. You'll be expected to use the 975 for volume control. If you're actually changing the ARC output level, or if the TV temporarily de-syncs the ARC output when you attempt to change volume on the Samsung, the audio dropout you're hearing on the 975 is because it's re-establishing the HDMI link.