"Are they in the subway?"
Translation:Они в метро?
It would mean "Are they in that metro?"
Also, the gender doesn't match, it should be в том метро; метро is neuter.
In a situation where there are several subway systems in one city, you could perhaps ask "Они в ЭТОМ метро?" (but you'd have to apply appropriate case pattern). But as long as there's only one subway usually in any given city, we don't need to differentiate it from any other subway, and "the" becomes unnecessary.
When the app says an entire sentence containing the word "в", she pronounces it as an "f" sound. But when she says the word "в" by itself, she pronounces it as "vway". This is confusing. Why does she do this? How is it supposed to be pronounced?
It would be a different meaning. 'HA' is equivalent to 'ON' in English and would expect explanatory sentence' end. Here was 'B'= 'IN'
In speaking language, in a question form, 'A' appears at the beginning of the sentence that gives the phrase a more completed sense; 'А они в метро?' (which is not accepted) :/
"А" is unnecessary here. It adds meaning to the sentence. You can ask a questions without adding "А" :)