It sounds like U
Before the letters BUMF - ב,ו,מ,פ
And before a letter with no vowel (Shva nach Nikud) like "גברים"
But modern Hebrew speakers don't use it often when talking it is for formal talk.
It seems like ב is "b" at the beginning of a word and "v" everywhere else. I don't know if it's like that all the time, but I'm assuming the "b" sound became a "v" when it became the second letter of the word instead of the first.
It isn't v in the middle of the word all the time, but your observation has merit: without any additional reasons, when a starting b becomes second, it becomes v. This does not apply to loan words not adopted into Hebrew.
there are rules associated with dagesh or emphasis. Basically the following letters change pronunciation at the beginning of the word in modern Hebrew: פ becomes "pa" כ becomes "ka" instead of "kha" and ב becomes "ba" instead of "va". If there were nikud (vowels and markings) the accented letters would look like this פּ כּ בּ
My hebrew teacher said, you can't have two "sh'vas" in the beginning of a word, so you change the v' to u' and then adjust the first consonant.
if i pronounce בשר ובירה as ba-sar ve-bi-ra (instead of ba-sar u-vi-ra), does it sound weird to a native speaker? or is it ok?
Beer is an alcoholic drink, alcohol is used in many parts of Judaism. And you are supposed to have meals with meat on the sabbath.