The poem "Night Voices" is a simple four-stanza poem that can be interpreted many ways. Although it starts innocently enough, with the children asking their father about sounds that seem to be voices in the night, the final two stanzas become progressively eerie as the children become more frightened and the father sits "so still and straight, / Ever staring, ever smiling, at the door." Certainly the poem is reminiscent of Edgar Allan Poe's "The Raven," which also begins with strange noises in the night and ends with a man sitting immobilized. Perhaps, as in Poe's poem, this father has lapsed into mental illness; the sounds are harmless sounds, but the man may have begun to believe that they are supernatural voices.
Another interpretation is that Father has nefarious intentions and that he has formed a league with the "night voices" that are suspiciously chuckling in the glen. The voices could belong to human beings--either crazies or criminals--who the father intends to allow in the house at the right time. Thus he is "waiting" an appointed time or signal and smiles in anticipation of what he will do with the friends who are soon to arrive. This interpretation is more disturbing than the first because it suggests harm may come to the children.
Alternatively, perhaps the voices belong to fairies or other supernatural beings. The father, having finally figured that out, enjoys listening to the other-worldly creatures. The problem with this interpretation is that it does not account for why the father "sit[s] so still and straight." This description casts an unnatural feeling over the father's actions.
Another explanation could be that the father has become bewitched by fairy creatures. Although at first he is able to explain away the "voices" to his children, in the third stanza he seems to not respond to their obvious fear, and in the fourth stanza, he appears mesmerized. Interestingly, Doyle was himself a strong believer in spiritualism, even to the extent of having fallen for obviously faked photographs of fairies (see link below).
Here's one last thought in a lighter vein: What if the father was playing a joke on his children? Perhaps it was Halloween, or he just wanted to teach his children not to be gullible. He may have gotten some friends to make the noises outside, and then pretended to slowly become mesmerized. If so, the next thing that would happen after the fourth stanza would be that the friends would come in, the father would laugh and say "Gotcha!" and the children would be embarrassed but relieved.