The (easy to miss) explainer for ABC says that beta is normally pronounced as V, but that's probably an oversimplification... https://www.duolingo.com/skill/el/ABC/tips-and-notes
In Modern Greek, αυ sounds like av or af, depending on what follows, so your ears are picking up the correct pronunciation (it's aftó in this case). This is explained in the Tips and notes section here https://www.duolingo.com/skill/el/ABC/tips-and-notes.
It was like 'ou' maybe a couple thousand years ago... There are a lot of resources for Ancient Greek online, make sure you include the term 'modern', not just 'Greek'. ;)