allthingslinguistic:

xkcd namechecking two linguistic effects: the Stroop effect (which is very well-attested) and the Sapir-Whorf effect (which is not, except in its weakest form and in very specific domains). 

The tonal parts aren’t related to a specifically linguistic effect but rather refer to the Doppler effect in physics, whereby a sound that moves sounds like it changes pitch. Naturally, in a tonal language, a change in pitch equals a change in tone, which would make it harder to understand.