If it's a lets play, no probably not. If you're doing the commentary after the fact and can stretch the speed of the video to fit the length of each carefully chosen song, and you are careful of audio levels to be sure it's not over taking your commentary (comm is the heart of a channel IMO), then yes, yes and yes, you should add music. After doing it a bit you'll get a natch for fitting the right length song to fit that part of the video's length and mood which can really add to the ambiance of a video with the entire goal being increasing retention time (for success) or for me I like the sound of music to game play, so win win.