The computing term BIOS, as I have heard it in the U.S., in commonly pronounced /ˈbaɪɔs/ or /'baɪoʊs/ — i as in fly (not bee) and s as in moose (not muse) if that is a help. The only people I know who pronounce the first syllable as /bɪ/ or /bi/ are non-native speakers.
The orthography and pronunciation of an acronym are contrived, as with most any neologism. The pronunciation of an acronym may have no relation to the pronunciation of its constituents— I might consider thinking so to be a variation of the etymological fallacy.
Consider three common acronyms:
- PIN (Personal Identification Number) – as /pɪn/, like the word pin, despite the pronunciation of identification
- scuba (self-contained underwater breathing apparatus) – as /'skuːbə/, despite underwater
- NATO (North Atlantic Treaty Organization) — /'neɪtoʊ/, despite Atlantic
Like so many other things in English, you cannot always predict the pronounciation from the spelling and vice versa. The United States Pacific Command, PACOM, is pronounced /'peɪkɒm/, not /pə'kɒm/ as might be suggested by Pacific or even /'pækɒm/ as might be suggested by Pac-Man. And while the FTSE 100 may be the footsy one hundred , the DJIA has no acronymic name; it's the Dow Jones or the Dow 30.
Someone who coins an acronym may well choose which letters to use in order to create a word, or to force it to conform to an existing one. Thus, the North American Aerospace Defense Command is NORAD, not NAADC, and the Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act of 2001 gained its ridiculous moniker so that it could be referred to as USA PATRIOT. For yet another military example, there's no inherent reason why HMMWV is pronounced as humvee other than that some Army official probably said it that way when the contract was being bid. In another universe it might have been a home-mauve or him-wave or something else entirely.
Still, the "official" way is not necessarily what gains currency in the population at large. Consider the longstanding discussion over how to pronounce Linux— some say /lɪnʊks/ and others /lɪnəks/, but in the earliest interview I ever heard with Linus Torvalds— perhaps from a time when his English was less practiced— it sounded distinctly like /linuks/ to me.