A few more details
You will note that four of the letters in the song can make two different sounds, depending on whether they are accompanied by a middle dot (called a dagesh) or, with the case of ש, where the top dot is located. There are actually two different things happening here.
With the first three letters in the chart below, the pronunciation depends upon whether the letter is beginning or ending a syllable. Given prefixes, suffixes and morphology (how words “morph” with different parsings) in Hebrew, this means that the same letter in the same word can sound like a b in one context and a v in another. But no worries, you can always know which to say based upon the presence or absence of the middle dot, and the meaning is completely unaffected.
On the other hand, שׁ and שׂ are in effect different letters. That is to say, a שׁ will never get changed to a שׂ or vice versa. Rather, the presence of one over the other indicates which word we are dealing with.