In the communities I participated in, that was just called copying, and generic MP3/MP4 player owners were often quick to point out that they could just simply plug in and copy files like a USB drive, whereas iPods needed iTunes and "syncing". I've never seen sideloading being used to describe anything other than to suggest "impropriety" or something that's not "officially unapproved", and only in the context of applications --- I haven't ever heard of someone "sideloading" music to a player either. I bet for the vast majority of others, this is also the case.