Off-Screen Speakers in Movies
Consider that the understandability of dialogue or monologue gets worse when the speaker is not seen on screen. A big portion of understanding spoken word comes from watching the mouth of the speaking person so if that is missing, it gets a little trickier for us.
You should take that into account when you write music for a voiceover scene and be even more careful to not overpower the speaking register of the voice with music. Handle the speaking voice as if it was an instrumental solo that you want to free up in your orchestration texture as well.