a chamber in a musical instrument, as the body of a violin, for increasing the sonority of its tone.
Origin of soundbox
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
the resonating chamber of the hollow body of a violin, guitar, etc
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012