You can have a script react to an onload event. I think it's just how implementations work today. If I get an electronic greeting card, so I have to download a media player just to hear some background beeps. In light of that, no difference. From an implementation viewpoint, sound is sound.
Background sounds and more important audio are different.
I don't see much difference between background and other audio. I'd agree that a sound is a sound and that programmatically we treat them the same, whether or not we bring up a player. Note that sounds played on load are synchronized with anything else played onload. But not important for unrelated sounds. The WG is saying that the UA needs to provide control over all sounds, even if all users wouldn't have the same level of control.