Your link says that Dolby Surround is one feature that won't work at all without MMX on, but doesn't go so far as to say that it's the only feature that's enhanced with MMX (it uses phrasing like "MMX‑dependent features such as Dolby Surround"). I'm not saying you're wrong, but your link doesn't confirm your statement either.

Doesn't look like there's any conclusive info out there, so probably someone needs to actually decompile it and look for MMX opcodes.