Fixed ear pads are permanently attached to the helmet, so you can't take them off when temperatures rise or swap them out for audio-compatible pads. They're typically found on budget-friendly helmets and models designed specifically for cold-weather riding.
The main advantage of fixed ear pads is simplicity—there are no parts to lose, break, or fiddle with. If you only ski in cold conditions and don't need audio or spring-weather versatility, they'll keep your ears warm just fine.
However, if you ski from midwinter through spring, you'll likely find fixed ear pads too warm on sunny days. And since you can't remove them, you won't be able to repurpose the helmet for biking or skating in the off-season. For most skiers, removable ear pads are worth the small extra cost for the flexibility they provide.