The grammatical form in Spanish that matches English's passive voice directly is very rarely used. Most of the time you simply make the subject disappear by moving the object into the subject position and then turning the verb reflexive.
So, for instance, you will frequently see: "Se habla español aquí." If you translated that in a stupidly-literal fashion, you'd get, "Spanish speaks itself here." But no native speaker means that, when they say those words. They mean the thing that we express as, "Spanish is spoken here [by people who speak]."
Reflexive-passive is very common. Far more common than anything like, "Español es hablado aquí [por personas quien hablan]."
So, first note that the verb is reflexive. This means that the verb is only going to be attached to one noun phrase for both the subject and object roles. So you know right there that "he heard a sound" can't be right, because "he" and "a sound" are two different things.
So then you ask yourself, is this a standard reflexive, where the object is a repeat of the subject? "A sound heard itself"? No, that makes no sense at all.
So it has to be the reflexive passive: There's only one noun phrase -- the object -- and the subject has been suppressed. "A sound was heard."
(And then there's a third distinct usage of reflexive verb forms that's semantically unlike either of these. I wrote about it at great length here: https://www.duolingo.com/comment/789137 )
I am little no a lot confused. When you mouse over "se oyó" it gives the answer (he/she/it heard). I ran the phrase through the 3 translators on SpanishDict and it gave me three possibilities "a sound was heard" "One heard a sound" and "he heard a sound" So now it is clear as mud.