I've never heard of such a thing.
Some music players (e.g. PowerAmp) do have their own internal preamp gain controls, so I guess you could turn that up/down to change the relative volume of that app vs other media outputs. That's the closest I can come. Personally I'd just press the volume control a couple of times.
I assume you don't mean play music while also playing the "how to" videos, as I'm pretty sure that won't work anyway (at any rate it doesn't work with a music player and the youtube app - I just tested it and it was one or the other).