V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

MMX
       Mobile Module PCI eXpress [specificiation] (Nvidia, PCIe)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

MMX
       MultiMedia eXtensions (Intel, CPU)


The Free On-line Dictionary of Computing (30 December 2018):

Matrix Math eXtensions
MMX
MMX technology
MultiMedia Extension

    (MMX) (NOT an acronym for "MultiMedia eXtension",
   according to Intel, but an Intel brand name) A set of 57 extra
   instructions built into some versions of Intel's Pentium
   microprocessors for supporting SIMD operations on
   multimedia and communications data types.

   MMX-enhanced processors are due to be released early in 1997.
   They will be fully compatible with previous Intel processors
   and software but software will only benefit if it is written
   to use the new instructions.  They can handle many common
   multimedia operations, such as digital signal processing,
   normally handled by a separate sound card or video card.

   (1996-12-21)