そう “seem” used for hearing

In a book someone hears a woman voice and says:

As far as I know, そう(な) is to say something seems based on visual information, and my grammars agree with this: things like おいしそうなケーキ. If linked to something you heard, そう is used to report what you said/heard, like 明日だそうだった, "I heard it's tomorrow".

Given the context, it seems to me that そう is the first meaning, since the character isn't reporting something she heard.

Is that そう the first meaning? If so, is that use wrong/unusual?

• The key is to think of senそうry information in general, not only visual – jarmanso7 Oct 14 '19 at 21:48