Have you tried playing with the tracking to see if the Hi-Fi indicator lights up at all? I’ve had the odd tape where the sweet spot for the Hi-Fi not to drop out isn’t optimal for the video portion.
There’s no Hi-Fi indicator light on this VCR that I can see, but it has a switch that lets you manually select “Norm” (linear track), “Mix” (linear plays in one speaker, Hi-Fi in the other), and “Hi-Fi”. I’ve tried it with several retail tapes now, and all three of those switch positions work as expected, and the Hi-Fi sounds great in all of them except for one, Night at the Roxbury, in which the Hi-Fi track has major interference going on, like it’s not tracking properly. The linear audio works fine on it though. But in the case of the Terminator tape, it is just dead silence on the Hi-Fi switch setting, and silence in one speaker on the “mix” setting. I adjusted the tracking all the way from the left to the right and back again, and it made no difference.
Note that the version of this release which is labeled on the sleeve “HBO Video” doesn’t say “Hi-Fi” on it anywhere, which you can see here:
https://imgur.com/a/kksbArY (scroll down on that page to see all three pictures)
I assume that’s the exact same release as the Thorn EMI-labeled one, because HBO and Thorn EMI were in cahoots, and my Thorn EMI copy says HBO Video at the start of the video, and after the credits, it says “Thorn EMI / HBO Video”.