Music is an integral part of our training in our dojo.
But the comment about serious is apropos.
It doesn't matter all that much what the music is, as long as it conforms to some rules:
1) Solid rhythm. The point of the music is to practice rhythm. To this end, my teacher will often turn the music off without warning to see if our rhythm falls apart. Let's say he gets a little grumpy if it does. But the music stays on pretty much non-stop in beginner's classes.
2) Careful about vocals. This isn't just about inappropriate lyrics (though that is a factor), it's also about distraction. If the music stops being a rhythm enhancing tool and starts being a distraction, it's no good.
3) When the music is on, it's on. When it isn't, just shut up and do your Aikido. IOW, the music has a purpose and when that purpose doesn't apply to the particular lesson, it goes away.
I'm also an instructor and I find I use the music or not depending on what I'm teaching that day. The character of the music may change depending on the lesson as well. Different music for meditation (if we use it at all) versus randori.
One thing I use the music for in kids classes (and sometimes it even works) is to cut down on talking. Play it just loud enough that talking is difficult, set them a difficult, non-stop exercise and yell, "No talking" occasionally during class. Even the most unruly kids eventually give up the attempt to talk.