Can you cast a creature spell for its morph cost face-down even if you don't have priority at sorcery speed?

Casting a creature spell face-down using its morph cost is still casting a spell, which means the normal rules for when you can cast spells apply. You must have priority to cast any spell (CR 116.1), and morph spells are cast as though they had no mana cost using their morph cost instead (CR 702.36c). However, they are still creature spells and are subject to sorcery speed restrictions unless something specifies otherwise.

Sorcery speed means you can only cast the spell during your main phase, while the stack is empty, and while you have priority (CR 307.1). No special exception exists in the morph rules that bypasses the requirement to have priority or that allows casting at instant speed unless the card itself has flash or another effect grants it.

It is worth noting that turning a face-down permanent face up (using the morph ability) is a special action, not casting a spell, and does NOT require priority (CR 701.33a, 116.2b). This is a common source of confusion — turning up is free from the priority requirement, but the initial casting is not.

Concrete example: It's your opponent's turn and they tap out to attack. You cannot use a morph cost to sneak a face-down creature onto the battlefield during their combat phase; you must wait until your own main phase with the stack empty and priority in your hands.