"become speechless" is correct and usable in written English.
You can use it when you are trying to express that someone was overwhelmed with emotion and rendered unable to respond. For example, "When he saw the incredible sunset, he became speechless."
Who says that we must use scientifically and technologically approved languages or become speechless?
We encountered certain people who themselves would become speechless, as they had never met deaf people before, much less done business with them.
Some poisons were slow in acting,104 others rapid;105 some caused one to become speechless until death ensued.106 Some bodies became black or livid after death.107 Dio108 tells the story of how Nero had the body of Germanicus whitened with gypsum, after it had turned black from the poison administered by him.
Become speechless, widen your eyes slightly, and stare at a fixed spot.
"I became speechless to hear that the county provided luxury houses at Mirae Scientists Street.
The driver of a sedan directly behind honked his horn; the cabdriver cursed, then became speechless as he looked back and saw that the protesting car was full of uniformed policemen.
There was a moment in this year's final when Brendan, a hot contender for the title, became speechless with emotion trying to explain why he had been inspired to bake a "family reunion" cake.
It is at this point in the story that my audience usually gasps in horror or becomes speechless, but these high school students were a tough bunch.
When you act corny you show your loved one that you care enough to drop all pretenses and make yourself vulnerable -- for her! Finally, when in the presence of your loved one on Valentine's Day, you may become wonderfully speechless.
When Radiolab reporters asked the professor whether she had ever thought about how her invention could be used to create videos of world leaders saying things they never said, she became literally speechless and eventually stammered out that her role as a computer scientist was to invent stuff and let others cope with the consequences.
Ada Anderson of The Ball State Daily News expressed the view that "Speechless" would become a popular song, and writers for the South Florida Sun-Sentinel stated that the ballad would take time to get used to.
