The short 'i' in the word "believe"?

Question

What is this merger called? I know there's the nearer-mirror merger, where both words are pronounced with the exact same long i, but which merger is responsible for the pronunciation of the short i in the "be" part of the word 'believe'? The singer is Dan Tyminski, who was born in Rutland, Vermont.

AVICII - Hey Brother

At first, I thought it was the southern drawl:

Stage 3 (/i/ → [ɪi] and /ɪ/ → [iə]): By the same pushing and pulling domino effects described above, /ɪ/ (as in hit or lick) and /i/ (as in beam or meet) follow suit by both possibly becoming diphthongs whose nuclei switch positions. /ɪ/ may be pushed into a diphthong with a raised beginning, [iə], while /i/ may be pulled into a diphthong with a lowered beginning, [ɪi]. An example is that, to other English speakers, the Southern pronunciation of fin sounds something like fee-in, while meal sounds something like mih-eel. Like the other stages of the Southern shift, Stage 3 is most common in heavily stressed syllables and particularly among Inland Southern speakers. Wikipedia

but there's obviously no drawling in there. Then, I thought it was the fill-feel merger as there's an L sound in "believe":

Lax and tense vowels often neutralize before /l/, making pairs like feel/fill and fail/fell homophones for speakers in some areas of the South. Some speakers may distinguish between the two sets of words by reversing the normal vowel sound, e.g., feel in Southern may sound like fill, and vice versa. Wikipedia

The fill–feel merger is a conditioned merger of the vowels /ɪ/ and /iː/ before /l/ that occurs in some dialects of American English. The heaviest concentration of the merger is found in, but not necessarily confined to Southern American English: in North Carolina, eastern Tennessee, northern Alabama, Mississippi, Louisiana (but not New Orleans), and west-central Texas (Labov, Ash, and Boberg 2006: 69-73). This merger, like a lot of other features of Southern American English, can also be found in AAVE. Wikipedia

And this seems like a plausible theory, but I don't think I'm right.

Answer 1

This is just the normal reduction of /i/ that happens when that phoneme is unstressed.

You have a continuum of possibilities ranging from [i] → [ɪ] → [ɨ] → [ə]. Native speakers will never perceive any of those to be anything but /i/ in the first syllable of believe.

The same thing also happens in words like receive, receipt, recede, redeem, redeemer, reject, release (but not re-lease), deceive, deceit, delete, behead, beneath, befit, bereft, detect, defect (verb), detest, determine, methinks, pretend, prevent, relent, renew, reveal, secede, serene.

You’ll notice that in the clip you referenced, not all occurrences of believe have precisely the same vowel there in that word’s first syllable. These are just different realizations of the same underlying phoneme, and the variation doesn’t matter in the minds of the speaker or listener.

Answer 2

It's possible that you have taken this far into inexplicable technicalities. Of course schwa is the most frequent vowel in English, and this is nothing more than schwa. A few speakers go to great lengths to utter the precise long e. But by far most just roll with what is comfortable, and what's comfortable is "ə", especially when established, cultural English dictates the pronunciation of the masses. How the architects of early English spelled these words was largely happenstance in any event.