President Harding's speech to the United States Chamber of Commerce took place on May 18, 1922. The photograph included below is from page 43 of "Radio for All" by Hugo Gernsback, which was published in 1922. It is the same as the one that appeared in the original article, but included here because it is much higher quality. (The Gernsback book incorrectly states that the photograph was taken at the Decoration Day ceremonies in Arlington, Virginia -- that event actually took place two weeks later on May 30, 1922, and was not broadcast by radio.)
Washington Post, May 23, 1922, page 5:
Harding addressing the Chamber of Commerce
    President Harding photographed while addressing the United States Chamber of Commerce in Washington. His address attracted unusual attention because he said that no other achievement of the Administration ranked higher than building up the merchant marine. The picture shows the sound detectors used to catch his speech and broadcast it by radio throughout the country.