In the movie, during a fight between Eddie Valiant and Judge Doom in the Acme Factory, when Doom reveals that his cane is actually a sword cover with a sword hidden inside, Eddie tries to find something to defend himself with, only to find a box marked: "Singing Sword." Thinking that this will be a perfect weapon for him to use, Eddie opens the box and takes it out, only to find that it is a "Singing Sword" toon (literally singing). It bears a striking resemblance to Frank Sinatra and starts singing one of his songs, "Witchcraft". Eddie and Doom are intrigued by this and pause for a while, but Eddie soon throws it away somewhere and they continue fighting. It isn't seen throughout the rest of the movie after this.
The "voice" of the Singing Sword was performed by Frank Sinatra (and may also be a possible caricature of him).
In the Bugs Bunny short Knighty Knight Bugs, there was a magic sword known as the Singing Sword because of its power to vibrate and hum a tune.