While there should be a better way to choose which volume to use, and changing the settings option of Change with Buttons to Off, there is a much easier way to control the volume of the media you’re watching instead of paying attention in meetings at work.
Simply put (I assume you need it simple since you obviously have a low attention span), turn your privacy on. On the iPhone, it’s the little switch over the volume buttons on top left side of phone.
Then, you can open your dancing piglets video, set the volume while it’s playing, then turn your privacy off again.
There. Have a good meeting. Hope they don’t ask you any questions that aren’t about princess penelope and the piglets of penzance.