The two sound exactly the same.
Only the context can help to determine wich is wich.
Here it is clearly “libérée” because the girl is singing how she is liberated and freed from her past, and not giving commands to liberate someone (or herself).
At least it is how I understand it 😀
In isolation, the words are pronounced identically. However, in context, there may be a liaison after the vous forms. Also, while not the case here, in classical singing (and occasionally in other types too), the e muet may be pronounced. For example, the listener in “Vainement ma bien-aimée” knows that Mylio is referring to a female beloved, not just because that is typical of the context and because ma is used, but also because he hears /bjɛ̃.nɛ.me.ə/. In such a situation, it would be relatively easy to distinguish between the feminine past participle and the vous form.
The forms can instead be distinguished by context:
There is no vous that is previously addressed.
The next line is Je ne mentirai plus jamais, so it would seem that Elsa is referring to herself.
(It is entirely reasonable that one may not be able to identify a word until further context is given later. That is often how we distinguish between homophones or how we decide which of the multiple meanings of a word is applicable to a situation.)