Yes, you are right: another meaning of 喂 is "hey!" in precisely the circumstance you have described, to call out to someone who may or may not be there. I have also heard 喂 as an expression of surprise, in reaction to an unexpected event, such as the ending of a magic trick; this sense of 喂 is kind of like, "huh?" like when you reach down to pick up your smartphone, but it is not there, where you thought you had left it.
喂 is also a term for 'to feed', as in feeding a baby food. In that case, it would be wei4, but the audio still keeps that when normally wei2 is used when it comes to saying hello. If 喂 comes up later in the course as 'to feed', then I would understand the inconsistency.
There is more than one way to say "hello," that is, more than one way to greet people, and some greetings are more appropriate than others depending on the context. That situation is the same in both Chinese and English. In China, 喂 is a customary way of answering the phone; accordingly, 喂 is translated as "hello" in English, as "hello" is a customary way of answering the phone in many places where English is the predominant language. Alexander Graham Bell, regarded as the inventor of the telephone, actually suggested "ahoy-hoy" as a way of answering, but his suggestion never really caught on (albeit, C. Montgomery Burns still follows that advice). If you prefer, think of 喂 as "ahoy-hoy," specifically, a "telephone hello," but 喂 has other uses as well, e.g., "to feed (livestock or babies)." There's even the phrase, 良心喂狗, "to feed (one's) conscience to a dog," or, figuratively, to be devoid of conscience, which might describe a sociopath. But, I'm rambling, again.