Is using music illegal in videos that are intended to be uploaded to the internet?

I am doing a few video game LPs with commentary, and to cover the absence of decent commentary in the first couple of videos, I have decided to insert music to fill the void, as it is rather bland otherwise. I don't want to get hit for copyright infringement, so I decided I'd search around on the internet first, and that yielded no results that helped my case. Now I'm asking directly in hopes of an answer. I have seen people using music in their videos and in video game LPs before and they've been up for years now, but I have been seeing conflicting results and just want to knwo for sure. If it is illegal, then is a good alternative to use other video game soundtracks from games that I have bought? Or should I just stick with using the game audio from the game I am recording?