Those are some really good points actually. Alpha is a very strange character. Unlike with Negan and the savior's, The Whispers started all this when they killed Jesus and then kill the others after beta and some of the other Whispers went after Daryl and Henry and Lydia. They never had a reason to kill anyone.
So then there is this... it sounds like a female voice, but it could be the voice of a light voiced male. As for females, it could either be Maggie, Jadis or someone from Commonwealth. Whoever it was didn't sound like they were in distress, and I wonder, even though Ezekiel left the room, there's a chance Judith could have heard that or whoever has the 3rd radio could have also heard it by chance. Eugene made three of them but since the kingdom was abandoned and have moved to Hilltop, they probably have two radios with Alexandria having one, unless Eugene has it.