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.