"I heard her."
Translation:Je l'ai entendue.
The word order of your answer is incorrect and makes no sense. It can be either Je l'ai entendu or Je l'ai entendue if this was a listening exercise however, as we can identify the direct object "her" only je l'ai entendue is acceptable.
The reason for the ending E is the direct object l' precedes the compound avoir verb so requires agreement.
Sorry, but Google Translate is an unreliable source for grammar. Take a look here for information about the use of object pronouns in French. https://www.thoughtco.com/french-object-pronouns-1368886
It occurs when the direct object it is expressed by the pronoun "la". However, as this pronoun precedes a compound verb that begins with a vowel, it is expressed as *je l'ai entendue" and agreement with the past participle is required.
If the sentence had been "I heard him", it would still translate as "*Je l'ai entendu". Note the lack of the ending "e" as that would be due to the masculine agreement.
It's because "her" is a direct object in this sentence and "elle" is not a direct-object pronoun. For third-person singular direct objects, use "le" (m), "la" (f), or « l' » when it is before a vowel. https://www.thoughtco.com/french-object-pronouns-1368886
Possible confusion of the term "passive". "Entendre" (to hear) refers to just picking up sounds. You can hear something even if you are not listening to it. "Écouter" (to listen, to listen to) is an active verb that indicates intention. Example, you may hear people talking (entendre) but not listen to them (écouter). So you may have switched the meanings around. Entendre may be passive but écouter is intentional. Je l'ai entendue = I heard her. Je l'ai écoutée = I listened to her.
You are referring to the T&N for Comp.Past. It gives many, many examples, including Je l'ai entendu → "I have heard him".
Your example was purely to show where adverbs (such as souvent ) are placed with using passé composé.
The lesson T&N sentences are grammatical guides not an answer key.