In review videos, where I discuss a bunch of different subjects, I don't use background music. I would need to find a ton of different songs that fit the tone of each different segment of the review, and it would take far too long and extend my already glacial production time by a ridiculous amount (partly because I'm super picky with music).
In videos about a specific topic, where the tone of the discussion largely stays the same, I do use music, usually between 2 to 4 different songs.
Sony Vegas has an audio volume adjuster for each audio track. You can just slide it or input the -db you want to find exactly what sounds good to you. I want the music just slightly audible, but never loud enough to steal attention or even get close to matching the loudness of the speech. If somebody was to listen at a very low volume where you can just barely hear my words, the music wouldn't be heard.
