Which is the right pronunciation (Liaison rules)?

Question

I studied French in school for seven years, forgot most of it in forty years, and relearn it now from an App. One thing I struggle with are pronunciations / liaisons in specific cases, for example:
Elle est en forme
or
Je suis allée
(I marked the letter in question as bold)

In the app, they seem to pronounce it sometimes this way and sometimes the other way - so my question is:

Are there clear rules when to liaison (pronouncing that letter) and when not?
How flexible is this in reality? Do I stick out like a sore thumb if I do it the wrong way around?

PS: I am aware of different pronunciations and spellings depending of the subject / object being male or female. However, specifically in the “Je suis allé(e)”, in the App, the male speaker pronounces the ‘s’, and the female speaker doesn’t, which seems to be the wrong way around (or random).

Answer 1

Both are optional liaisons so the conflicting pronunciations you heard were correct. In casual conversation, I guess most people won't make them while in a careful or formal situation, there is more chance for people to do it.

The gender of the speaker doesn't affect in any way these liaisons.

See To what extent are liaisons optional? Can I have some examples?

Answer 2

Liaison between a form of être and the predicate is categorized as liaison facultative, "optional liaison".

Here's the Wikipedia article on liaison in French — the French version is, understandably, much better than the English one — where you can see this type is listed as the first kind of liaison facultative.

Answer 3

Je suis allée

I'm a native french speaker and I'd say that we'll make this liaison everytime. But it wouldn't be shocking not to say it in this case.

However, if the liaison isn't done in this sentence, it could be understood as "Je suis hâlé", which is sometimes used for "I'm tanned". Of course there would be no doubt if the complete sentence is "Je suis allé à Bruxelles".

Not making a liaison is usually not a problem in french, but what is considered a mistake in french is to make a liaison when the word start with a letter "h". "Je mange des haricots", "Un gros hélicoptère", "des halos lumineux" : never do the liaison on these ones.

But making the liaisons shows you've got a certain level in french language, but you've got to know the last letters of your words to make the correct liaison. It's not trivial as many french words have a last letter which is only pronounced when there's a liaison.