I’m trying to understand what it is that you’re looking for here: an onomatopoeic word that means the same as a term that means something that can’t be touched, seen, or felt? Because “morally bankrupt” itself is a figurative take-off on some more basic word or meaning. It’s a term that doesn’t have a literal meaning – and as far as I know has no characteristic sound, either.
Take @janbb‘s “debased”, for example. That’s a common word that people should know, a fine, useful and perfectly comprehensible word. “Morally bankrupt” is a flight above that. That is, “moral bankruptcy” is a figurative way of expressing immorality, the more fundamental term. It’s a figurative expression, since there is no literal way to be “bankrupt” in a moral sense, bankruptcy itself being a term applying in a literal way only to finance.
Not that there’s anything wrong with that (the use of the words, that is), and @janbb‘s example is correct: Debasement ≈ Immorality = Moral bankruptcy.
There’s nothing wrong, either, with using the figurative term “bankrupt” in other senses where it conveys an empty purse or vault: we can be bankrupt of ideas, for example, and “moral bankruptcy” is itself a commonly used term (unfortunately so, in fact, but only unfortunate in that the concept is so commonly evidenced). It’s a good term.
So far, so good. But now you’re asking for onomatopoeia, too. I like onomatopoeia (writing it, not so much). And I like how many common words have the quality: “slice”, for example, or “chop” (as the words apply to preparing food, anyway), and “drip” as it applies to water leaking from a faucet. Even “faucet” has a certain onomatopoeic quality when applying it (as a noun) to the sound of the aerated water that can gush from it. But that’s because the words sound like an action or thing that produces sound, or the sound of the thing itself – which is the basis for onomatopoeia, after all.
What’s the sound of moral bankruptcy? Give me the sound, and I’ll look for the word.