Okay wellllll .... not so simple

First things first, I was alerted to this issue because 2 HiQ instruments (drum + other) were silent when going back to rework a previously complete song. For information it was created in BIAB 2017, and my installation would have been through appropriate updates and then the change to 2018 plus updates to that since.
The instruments shows visually as playing in the mixer section (flickering sound ribbon) but are not heard.
I can no doubt substitute RT instruments but that's not the point, I expect there will be other instances the same waiting to be encountered, and I don't want to have to sub every time before I can even start making progress.
OK. So then ...
Noel, thanks and I started following your steps, stopped very quickly as I don't have a Program(x86) folder. I have checked by unhiding all/system/protected folders and nothing shows.
However, I have a 32 bit Win 7 installation which I understand means there's no Prog(x86) folder anyway.
Regardless, a full system-search for "Aria" finds nothing. Also, Aria is a 64-bit utility as I now understand it anyway, Yes?
However, I am unclear then how BIAB can be requiring Aria - which seems not to be there for it even to know about - to be initiated in order to be able to see/play instruments/voices that it was perfectly happy with before.
The "can't find" popup also asks if I "want to uninstall this product" (presumably Aria). I've always said no to that, partly because I don't want to uninstall anything indiscriminately, also because maybe it is needed regardless and just needs to be given a way to communicate.
Also, asking if I want to uninstall it suggests that it IS installed after all! Puzzling!!
Moving along:
Although there's (allegedly!) no Aria .dll file to point to, I did run through the second part of your steps just to explore/learn a little if nothing else.
At that point, when I clicked a track in the plugins dialog to see if anything relevant is shown at the "Add VST Plugin" stage (I had selected the missing/muted instrument), the Svorzando UI opens along with a popup saying the instrument from (in this case) Bank 2050 can not be found. I presume Sforzando relies on Aria too, and also that this will therefore have further implications of its own if not dealt with.
So, right now I can't work with something containing Hi-Q instruments, BIAB is wanting to know Aria's .dll so it can use a utility it seems to be relying on and has never had a problem with but which Windows says doesn't exist, and everything worked/played perfectly on this song perfectly around 6-9 months ago!
So:
Is Aria installed automatically by/within BIAB? If separately, should I install/reinstall it to see if that helps?
Do I even need that however in a 32 bit system? If I do, will it look for a x86 folder and I need to do something in advance without upsetting Windows?
If I don't need it and don't have it, why is BIAB unable to go forward without it now when it was fine without it from 2004 to some months ago?
If I do need it and do have it, where is it if BIAB appears to be aware of it but Windows doesn't?
Workarounds? I can always "yes" to the uninstall request, though I am fearful of what Windows would be uninstalling thinking it was Aria!
Or maybe like Windows, there are times when only a clean reinstall of BIAB will fix things?
Hope it is a lot simpler however!
BUT, before finishing I want to go right back to the original fault window ... "Aria Engine: Band-In-A-Box sounds. Can not find instrument Bank File at F:/bb/HiQinstruments/etc ..."
That Instrument Bank File however is exactly where it is supposed to be on the C:/bb installation, do I HAVE to go through Aria to get BIAB to look there for it or can I do that directly? Would it resolve the issue by reversing in to that connection that way and independently getting BIAB to see the file it wants?? If so, I need to be shown the steps.
I am really much too close to all of this now and possibly missing something very obvious, but I've got nothing else.
So for the moment I can't get on with a project, and that's not especially helpful with a couple of deadlines on the way ... fortunately not a problem for a few days yet anyway.
Can you/someone please point me to what I need to tinker more with under the hood to diagnose/fix this.
Thanks!