I just taught this to my students today!
The rule is that a vuv with a dot on top can either be a cholom malei (“oh” sound nekuda for the previous letter), or it can be a vuv with a cholom chaser (“voh” sound). But a vuv with a dot in the middle is a shoorook (“ooh” sound), but it will NEVER make a “vooh” sound. It is just a vuv with a dagesh inside. So the word says “havos,” and the first vuv is a vuv with a dagesh, and the second vuv is its nekuda – a cholom malei.