Green’s Dictionary of Slang

1. in prizefighting, a solid blow that fig. ‘reverberates’ as it hits [a resounding blow].

1847		Bell’s Life in Sydney 13 Feb. 2/2: Sparkes finished with the left [...] catching in return a sounder on the ribs.
1848		Bell’s Life in Sydney 18 Nov. 2/4: Dick planted a right-handed sounder on the ribs.
1851		Bell’s Life in Sydney 4 Oct. 1/2: Donnelly stepped in and got his right a sounder on Haye’s ribs.

2. (Aus.) an ear.

1904

Sydney Sportsman (Surry Hills, NSW) 14 Dec. 1/2: [He] curls round the back of an unsuspecting punter [and] whispers a dead cert. into his sounder.