I know it is only a very limited step towards this but what I do in the situation where I suspect I might need “with-and-without” versions of a lot of soundscapes is to build a custom sound set for each “base state” environment.
I’m in the process of setting this up for my next campaign. I’m starting with:
on the roads
in the woods (safe)
in the woods (scary)
in the ruins
in the catacombs
and a few more like that.
These I try to keep quite stripped-back, containing only the ambient sounds to simulate the physical environment and maybe a basic level of mood (hence the good/bad safe/scary alternates). I build this basic set myself starting from predesigned moods and adding to a custom soundset as I go.
I have these set up on my StreamDeck as buttons to trigger these moods.
Then I set up a row of buttons with music to add on top of the moods - that goes a very long way towards establishing the emotional tone. I’ve mostly built my own playlists here because I’m paying an unfeasible amount of money for a broadcast quality music library for my actual business, so I want to extract value from it. But it would work just fine to use a selection of Syrinscape music too.
Then I set up a row of buttons with single elements that most vividly reflect a big change in environment. I know it’s not using the full power of Syrinscape but it’s often possible to find a single Rain or Wind or Fire SFX loop to add that to the mix.
Finally I have some buttons to add the sort of foes I most expect to be around to the mix: even in a sandbox game I often have SOME idea of what the possibilities are for this session. So I might add buttons for ogre fight, ooze sounds, bugbear roars. If I know there is a possibility of a bigger fight, I might build a mood for that concentrating on battle sounds and enemy sounds and not really caring too much about the ambience - I figure if you’re fighting a vampire, that’s the thing you’re paying attention to, your awareness of the stone clinks and water drops in the background probably goes anyway. So I’ll make that with quite an empty backing soundscape, and rely on the “one dominant mood” SFX if I want to keep it emphasised that you’re fighting a vampire in the rain.
Admittedly, changing moods does reset all of these extra elements, but with them set as buttons on the StreamDeck it doesn’t take more than a tap to re-add them. I don’t think it is a complete de-immerse, personally, since there’s a traditional scene transition in such moments both in tabletop and in VTT play where you bring up the new map or describe the scene.
What I’m currently stuck with is an annoyance where I can’t find a way to control the volumes of these additional elements in the mix when triggered via StreamDeck. (There’s a way of doing it on PC, but I’m on a Mac. Sadface.)
To organise this all effectively, when I do my prep I add everything and the kitchen sink to a campaign (or part-of-campaign) specific sound set. The new search beta has GREATLY improved the ability to do this by letting you audition sounds and click “+” to add to the current soundset. Even when I add an unexpected sound on the fly, I find it via search and click to add it to the campaign - if I’ve used it once, I might want to use it again. And it means the volume controls for all the elements are there on the Syrinscape screen, so I can quickly pull down something that’s too intrusive.
It’s not perfect - as the OP says the main issue is that the GM is dealing with a massive cognitive load in the game already. But unless your campaign properly hops from future towns to prehistoric desert to 9th plane of hell to the inside of a chocolate factory without any prior hints to the GM, I’ve found it possible to get 80% of the way to what I’d want from having a separate DJ with 20% of the work, and that’s always my aim!