DL's translation seems to ignore the "Da" here. Is it that redundant? Wouldn't "Here comes someone" be more appropriate?
I think the sentences are pretty equivalent like they are. "Da" is used to point to the direction from which someone is coming, if that someone is visible (but not recognizable) ("Sieh mal, da kommt irgendwer. Erkennst Du, wer das ist? Kennst Du ihn?"), but the sentence could also be said if you just hear someone or something ("Hörst Du das? Da kommt irgendwer!"). Do you have a more idiomatic expression for this in English?
The expression I'd use, as a native Brit, would be "there's someone coming". The "Da" being represented in the word "there", and making a more reactive expression than DL's plain statement.
Just keep in mind that "da" doesn't" always refer to a (physical) place, i.e., the sentence here doesn't contain more information than "Jemand kommt"/"Someone is coming".
With short colloquial, idiomatic expressions like this, translation is more about swopping an idiom in one language for one that you'd use in the same circumstances in the other. I think you are more likely to use DL's offering than to actually literally translate "da" as "there" in this one ............. but it's not wrong!
The male voice says something like "Er kommt irgendwer." No way he says "Da".
The female voice is sparkling clear.
My guess is; it comes down to "Idiom": that strange component thrown in by time and custom that makes English, particularly, a pig-of-a-language to learn. There's no small component in German too.
Here comes someone, there someone is coming, someone's coming ... all shot down. And no explanation. Just a bunch of us guessing why.