I'm having a little trouble too...We know er=is, den=it, der=there, so nede must=down.
The word-by-word translations would be "Is it there down?" but I think the key is that when you hover over "den der nede" one of the translations is "it down there"
My guess is that this is one of the phrases that we just need to recognize rather than translate word-for-word.
Because the "it" being referred to might be gendered. It's not referenced in this short sentence, but when someone is asking "is it down there?" then "it" has been established prior. If they're talking about a car (en bil), then "den" would be appropriate, but if it's a train (et tog), then "det" would be the choice. That's the way my Norwegian explained it, anyway. I think Duolingo just wants us to know that either "det" or "den" may be used for "it", so that both will seem familiar when we come across it.
"Er det der nede" would also mean the same thing. I think the only difference is if the "it" you're referring to is a gendered word. So if you're talking about an orange "en appelsin," you would say "er den der nede." because it's masculine. But if you're taking about an apple "et eple" it's neuter so you would say "er det der nede."
I'm just learning still too though, so please correct me if I'm wrong :)
Unfortunately, course contributors are limited to disabling the audio on select sentences (type-what-you-hear); we don't have the ability to "fix" them and we can't disable all the sentences that contain "den."
On the bright side, I started learning Norwegian a few years ago and I became accustomed to the pronunciation early on, so it sounds normal to my ear.
I just encountered the same problem in another practice sentence, "Den kua er hennes" (15942330) – at slow speed, of 'den' only the 'n' is left. So I gather that one audio-version of 'den' is used for all example sentences, and all sentences containing 'den' will have this problem? I hadn't really been able to make sense of that part of your answer before (disabling all the sentences that contain "den"), but this would explain things.