When placing a song in a film, not only does the music have to support the mood and story, it also needs to leave space and room for vocal narrative. There is an art of arranging music to voice; Frequency and Register in a composition plays a large role in helping a voice-over stand out more and have clarity. Like most other factors in music placement, the role of music is aimed to compliment not only what is being seen on screen, but what is being heard.
Most often, if the music is playing in the same register as the voice-over, there's a good chance that it will be hard pick out what is being said and what is being played, this can become a little muddy. For example, if the voice-over has a higher-pitched frequency, and the music is playing in the same register and pitch as the voice, neither the voice or music will have clarity. When considering finding a song that compliments a higher pitched voice-over, think about one that supports the voice with instruments playing in lower registers, music that creates a balance. Here is an example...
We recently collaborated with Travel Oregon on their vivid and brightly colored vignette about
where we get our food in our thriving cuisine culture.
Throughout this video, there is a vocal narrative that travels along with the imagery. The voice-over in itself has a more mid-range level (not too high, not too low) , so when thinking of how to compliment the voice, the music needs to leave space for it to shine through. When taking note of the soundtrack, the music is playing in the higher register (shiny guitars, singing) and leaves room and supports the voice-over.
Like anything is storytelling, finding ways to bring elements together to create harmony and clarity makes for the best story. Music and voice are different elements and when they leave space for each other, they find that harmony and clarity to make the larger story have the impact that you want to convey.