Speakers were devices for enhancing the volume of sound.
In South Wales in 1959, Billy built a speaker out of leftover parts from World War II; he used it when he performed with The Lorells. The Seventh Doctor and Billy adapted the speaker to project the singing of the Chimeron princess. This disorientated the Bannermen. (TV: Delta and the Bannermen)
Nanogenes created by the Chula would give a subject "Om-Com", the ability to project their voice through anything with a speaker. (TV: The Empty Child / The Doctor Dances)
The Tenth Doctor used his sonic screwdriver to enhance the speakers at the reception of the failed wedding of Donna Noble and Lance Bennett, creating sonic vibrations powerful enough to make the attacking Roboforms fall apart. (TV: The Runaway Bride)