I second that (native speaker).
There is no way to tell them apart when pronounced properly in Russian, full stop - i.e., not just in this audio. With the exception of few very local dialects, an unstressed "o" is basically a schwa sound (https://en.wikipedia.org/wiki/Schwa), same as an unstressed "a".
And your interpretation of the difference between "Ты хочешь пива?" (genitive) and "Ты хочешь пиво?" (accusative) is absolutely correct as well. Btw, the latter could also mean "Would you like the beer" - I could be handing you a bottle while asking this question.