The difference is - in one instance, the particles > sound, it's using ambient particles - ones already there.
In the second, a space fleet is actively expending energy (and surely gobs of money) to dump artificially generated air into the surrounding vacuum.
Does that sound entirely brilliant to you? Even surrounded by a shield? The air costs energy and money to produce, and the shield definitely costs energy. All as an elaborate expedient for fighter pilots to hear sounds?
Fighter pilots are considered expendable resources in most SF universes, so I wouldn't think that the expense would be either justified, or sane.
Do you see yet why people wonder what the heck you're talking about? This is elementary science class stuff. No, you don't pump air into a vacuum, so you can hear the sounds. That is not very smart.