No Nightingales
What I think Crowley thinks he's saying here is that he's been using their coded language-- speaking in Nightingales-- for basically this whole scene and Aziraphale isn't hearing what he's saying and he really, really, really needs to hear it in Crowley's mind because Crowley thinks the (really non-existent) threat of The Book of Life is looming. He thinks if Aziraphale goes with The Metatron that Aziraphale will be erased into a state of non-existence and Crowley's proposal is actually a plan to stop that from happening.
Crowley, by the "no nightingales" portion of the scene, is desperate, and his last play is to, in his mind, directly refer to their secret language using what is obviously coded language by saying nightingales directly and just hope that The Metatron takes it as some sort of in-joke and not a coded message.
He asks if Aziraphale can "hear that" when no sound exists and points up as a way of making the point that Aziraphale isn't hearing the wordplay in what he's been saying. He is actually referencing two types of birds at once-- the nightingales of their hidden language and the crows from their first, fumbling attempts at forming what would become their nightingale speak back in Job's courtyard and from which Crowley took his name.
He's referring to this:
He's trying to directly reference the moment that he let Aziraphale hear the crows bleating like goats and Aziraphale realized the true meaning of what Crowley had just said the moment before. He's trying to say that something like that needs to happen again, even if the end result of this scene is, ultimately, that it does not.
Crowley is trying to get Aziraphale to rewind mentally for a moment to everything that Crowley has said previously in this scene and, instead of taking it on a surface level, run it through the filter of it being wordplay in his mind so that he'll understand what Crowley was trying to say.
Seems like a smart move, right? How could there be more confusion if the word being referenced is the word they use to refer to the language in the first place?
What Crowley doesn't take into account is the fact that he's saying this while being about to walk out while they're both upset and that, because nightingales is the secret language shorthand word? It *also* means-- to both of them-- their love for one another.
So, what Aziraphale sees and hears here is not baby, please listen and realize that you didn't hear the real words I've been saying...
What Aziraphale sees and hears here is:
You don't love me. You and your Heaven bullshit-- there were never any nightingales. This was never love. It wasn't real. It's over.
And Aziraphale?
It would have been less painful if Crowley had physically ripped Aziraphale's heart straight out of his chest.
He's heartbroken and he's furious and every emotion along that spectrum because-- funny story-- Crowley hasn't heard a bloody word in nightingale speak that Aziraphale has been saying in this scene, either.
A series of unfortunate events fucked this conversation from the start because the beginnings of it involved things that made them both assume that the other wasn't using the language at all.
They each thought the other didn't see Coffee Dude watching them as a threat and started to try to convince one another there was trouble and what they each thought that trouble was and what their plans were to overcome it. They have been only taking one another's words on the surface level and so they don't really understand what one another has been saying.
So, Aziraphale hears "no nightingales" and he's thinking of all the human art into which he's put the word for their language and their love because, to them, that is the same thing... the centuries of mythology and stories and poems and the part of Romeo & Juliet about the secret nights spent together and their damn song... and how dare Crowley say this isn't love?
They've spent millennia loving each other with their secret birdsong language and now, to Aziraphale, Crowley's wielding the one word that means to them just that as the weapon to end them. How dare he use that of all words to say that this love isn't real and that whatever it was is done when it means thousands of years of singing that love to one another?
You could torture Aziraphale for a millennia and he would find it less painful than what he thinks Crowley just said.
So, ya know... when Aziraphale thinks what is happening is that Crowley just went from "marry me" to "you don't love me and it's over" in about four minutes flat with a waking nightmare of misunderstandings in between and is now about to leave him just when Aziraphale needs him most...
....maybe the reason why Aziraphale is taken aback by the kiss is not because it's the first but because he's just got emotional whiplash and this is an unexpected last kiss that is coming seconds after, in Aziraphale's mind, Crowley having just accused Aziraphale of not loving him.
Yes, their exactly's are not the same here. And maybe Aziraphale's face in the elevator is him finally getting a chance to think about what Crowley was actually trying to say and (hopefully) starting to figure it out.
That's become my feeling about the elevator, too. He's clearly thinking something through in the mid-part of the elevator shot and then the end expression is 'fuck' on a cosmic level lol so he might be realizing some of what Crowley was trying to say... which is brutal, actually, because he's probably about to lose his memories for a spell in the story here. Not for long, likely, but maybe until he's reunited with Crowley. (I'm pretty sure The Bentley is Aziraphale's fly, made unintentionally in S2.) But that means some of his last thoughts, basically, are going to be about That Scene and knowing where it all went wrong and okay, I have to stop depressing everyone...
When in doubt, the flirty vol-au-vent gif:
There, all better.