Today one of my players asked me, for future reference, if speak with dead could be used on a door that has a face, including a mouth, carved into it, on the grounds that a door is just dead wood, and speak with plants is already a thing. My thinking is "no," because I think the spell implies a functional (or once functional) mouth, but...I kinda wanna say yes, because I think it's an awesome idea. How would you rule?
I think I'm gonna say yes, but only if speak with plants is also active.
There are problems. The 2024 text says...
"You grant the semblance of life to a corpse of your choice within range, allowing it to answer questions you pose. The corpse must have a mouth, and this spell fails if the deceased creature was Undead when it died. The spell also fails if the corpse was the target of this spell within the past 10 days.
Until the spell ends, you can ask the corpse up to five questions. The corpse knows only what it knew in life, including the languages it knew.
...This spell doesn’t return the creature’s soul to its body, only its animating spirit."
Trees are not generally considered creatures, and trees don't know any languages in life, and don't generally have animating spirits. Treants and other plant-creatures, yes, but not normal trees.
Speak with Plants gives some limited sentience and animation to living plants, but doesn't work on dead materials. The door is not a valid target for the spell.
So, if you want me to be picky, no, I don't think this works. If there's divine magic involved, and the results are very specifically in service to the divine power behind the attempt... maybe.