Sefer Ohel Shlomo (page 36) relates that a relative of the Tiferes Shlomo zy”a had a child who was mute. This relative was once visiting the Rebbe before Shavuos and a Machzor was sitting on the table. The Rebbe opened it and told the child, “Say the words: ‘Akdamus Milin V’Sharios Shusa!”
The boy immediately began to say the words. From then on, he was able to speak.