Have you heard the voice again since? If it continues, you may be hallucinating, it may be withdrawal from the alcohol, or it may just be a fluke. Do you live in an apartment or close to a neighbor or sidewalk? Sometimes voices carry and will sound like they are in one place when they come from somewhere completely different.

You were right in that it is not possible for any of us to determine what it is. Just keep an ear open to see if it happens again, and report it to your pdoc if it does.