When placing fluids into the world using buckets, the place sound is determined by the #minecraft:lava fluid tag. If the fluid the bucket contains is in the tag, then the place sound will be minecraft:item.bucket.empty_lava. Otherwise, it will simply be item.bucket.empty.

However, this condition does not extend to cauldrons. In particular, water buckets will always use item.bucket.empty, and lava buckets will always use minecraft:item.bucket.empty_lava. This behavior the exception compared to other forms of fluid placing (through players or dispensers).