"The Voice" by Thomas Hardy is not a particularly complex poem to understand, it seems to me. Understanding just a bit about Hardy's own life will add to your understanding. The speaker, probably Hardy himself, has obviously loved a woman with complete love. It is likely Hardy mourning the loss of his wife of forty years, Emma. She is the one, he says, "who was all to me." He is standing out in the open moors and believes he hears this "woman much missed" calling to him. If it is her, he asks her to reveal herself to him:
Can it be you that I hear? Let me view you, then,
Standing as when I drew near to the town
Where you would wait for me: yes, as I knew you then,
Even to the original air-blue gown!
This wistfulness is a hope that she will be restored to him in her original beauty--thus the "original air-blue gown."
In despair, he soon understands it's probably nothing more than the wind traveling "across the wet mead" and "oozing thin through the thorn from norward." The last image we see of this heartbroken, faltering man is leaves falling around him, wind oozing through the trees, and the woman, his wife, calling. Even knowing she is forever gone, he continues to hear her voice.