Background music can do wonderful things for your video. It can help create emotion, drive the pace and flow, and even hide pesky audio edits. But counter to what you may believe, the most successful background music is the music that you didn't even know was there.
If the volume is too high, the music will overpower the spoken narrative of your video. This… is no good. If background music is too low, it can paradoxically draw attention to itself by making the viewer strain to hear it. The goal of background music to invisibly assist your video, not create a distraction.
Mixing the music volume in your video takes practice, and there's no exact formula for what level the music should be relative to the voice. It's all about training your ears to feel when the music is sitting just right in the mix.
To start your training, play around with this interactive volume video and try to listen for when the volume sits right in the mix...