How do you pronounce the following set of 'words'?
I think pronunciation is related to region, laziness and the presence of phonetic pronunciation. 


Short acronyms are regional - Several answers here indicate this by the variation in SPI pronunciation.
Longer acronyms (4 letters or more) are generally pronounced phonetically due to laziness if they have a phonetic pronunciation. Some examples:


Longer acronyms with phonetic pronunciation to some extent


UART - Yu - ART
ASCII - AS - KEY
VHSIC - VEE - H - SIC


Acronyms with no phonetic pronunciation


VHDL - VEE - H - DEE - EL
XKCD - EX - KAY - CEE - DEE ;)


My pronunciation:


SPI - ES - PE - EYE
I2C - EYE - squared - C
LED - EL - EE - DE
USB - YU - ES - BE