It's useful to think of it this way: To utter means to make a sound; to speak means to make an intelligible sound.
I'm being deliberately simplistic here just to underscore the main difference. The truth is a little more complicated.
NOAD:
utter 2
verb [ trans. ]
1 make (a sound) with one's voice : he uttered an exasperated snort.
• say (something) aloud : they are busily scribbling down every word she utters.
So utter also means to "say aloud"; but speak has a more precise definition. (And one wonders why "aloud" is needed at all in the above entry.)
speak |spēk|
verb ( past spoke |spōk|; past part. spoken |ˈspōkən|) [ intrans. ]
1 say something in order to convey information, an opinion, or a feeling : in his agitation he was unable to speak | she refused to speak about the incident.
Not that this is always the case. Every word you speak aloud is an utterance, but not every utterance is of informational value.
curtailed, lost, ruined, squandered, nullified, wasted, rendered ineffective...
Best Answer
I think both are wrong, if one equates use with acceptability.
In medicine, we say "he lost his faculty of speech", where faculty is the ability, power; an inherent capability, power, or function; a natural aptitude. Googling ___ gets ___ hits: "he lost the faculty of speech" (14.8K), "he lost his faculty of speech" (13K), "he lost the faculty to speak" (6), "he lost his faculty to speak" (1).
Facility is an ability to do or learn something well and easily; a natural aptitude. Googling: "he lost his facility to speak" (7 hits), "he lost the facility to speak" (0 hits), "he lost his facility to speak" (7 hits), and "I lost my facility for speaking* gets 3 hits, two of which are this question.