It sounds odd to say it like that. English doesn't tend to use the definite article with abstract words like "heaven", whereas Swedish does. You can see another example of this if you look at "life" vs. "livet" :)
Although 'the heavens' is someimes used in English... I suspect it's some Mediaeval concept of there being several levels to heaven or something (c.f. 'highest heaven')
It's actually because Old Testament Hebrew frequently used the plural form, which then carried over in translation. The meaning is something rather like "the many parts of the enormous heaven" rather than "the multiple heavens".
You have the same concept in the word "Jerushalaijim", Jerusalem. The -aijim is some kind of plural ending, probably hinting at Jerusalem as a city of the people and a city of God.
You got me curious, so I checked. It's the dual suffix, but it is apparently a heavily disputed issue, as this reading for Jerusalem is a very late development.
Why is "The angels singing in heaven." wrong? (Oops, I just realized I had a typo on "angels" but "singing" is the word highlighted as wrong).
The Swedish sentence was in the present tense, so the English sentence needs to be too. This means it could be either "The angels sing in heaven", or "The angels are singing in heaven".
Your sentence is using singing as a sort of adjective, describing the angels as singing. This makes the sentence mean "The angels (which/who are) singing in heaven", as opposed to some other angels that are either not singing at all or singing somewhere other than in heaven.
An example context of this English sentence could be:
Q: "What is that awful noise?" A: "It's the angels singing in heaven."
I'm not sure how to translate that meaning back into Swedish, you would possibly need to use a present/active participle, something like sjungande, and/or a conditional pronoun 'som' somewhere. A native or more advanced learner could help maybe.
"It's the angels singing in heaven" would be "Det är änglarna, som sjunger i himlen" in Swedish. There are other (older) ways of saying it, but this is probably the one most Swedes would use nowadays.
Does Swedish also differentiate between heaven and sky or is it more like German and uses one word for either?