I2C, SPI, and USB have no set pronunciation. NXP/Phillips, Motorola, and the USB consortium declined to set a pronunciation in the relevant specifications for each of the protocols. They also decline to state whether they are acronyms or abbreviations. As such, there is no set guide for how they should be pronounced or used. It is left to common usage.
Personally, all four are abbreviations, and I pronounce them as such. I2C as Eye Two Cee instead of Eye Squared Cee, though Squared is more accurate. I have heard SPI called the Spy Bus, but not often. And that was a bit conflicting as there is a Spy Bi-Wire bus used by Texas Instruments, normally referred as SBW or Spy.
Basically, you pronounce them like others you talk to pronounce them, and never bring it up in polite conversation.