Evan’s suggestion does seem the most plausible of the senses offered by the OED. Definition n2 is below. n1 is to do with wild swine, while n3 is to do with sounding depths of water.

Tom

1. One who makes or utters a sound or sounds; one who causes something, esp. an instrument, to sound.

1591   R. Percyvall Bibliotheca Hispanica Dict. at Tañedor,   A plaier or sounder of any instrument, cantor.

1648   H. Hexham Groot Woorden-boeck   Een Luyder, a Ringer, a Sounder.

1809   ‘D. Knickerbocker’ Hist. N.Y. II. vi. iii. 100   The illustrious sun..did dart one of his most potent beams..upon the refulgent nose of this sounder of brass.

1831   Scott Count Robert ii, in Tales of my Landlord 4th Ser. II. 14   In the front..stood the sounder of the sacred trumpet.

1859   Dickens Tale of Two Cities ii. i. 34   The sounders of three-fourths of the notes in the whole gamut of Crime, were put to Death.

1591—1859(Hide quotations)

 2.

a. A telegraphic device which enables the communications or signals to be read by sound.

1860   G. B. Prescott Electr. Telegr. 91   Since the adoption of the method of reading by sound, another apparatus has taken the place of the register, or recording apparatus, called the sounder.

1872   F. L. Pope Telegraph iv. 32   The Sounder consists simply of the electro-magnet, armature and lever fixed upon a base.

1876   W. H. Preece & J. Sivewright Telegraphy 246   The Sounder, on account of the extreme simplicity of its mechanism, is less liable to faults than any of the other forms of instruments which are employed.

attrib.

1875   E. H. Knight Pract. Dict. Mech. 2247/2   Sounder-magnet, the magnet which operates the sounder in the receiving apparatus.

1860—1876(Hide quotations)

b. A telegraphist who operates or has experience with this.

1887   Daily News 2 May 7/3   Telegraphist (sounder) desires engagement.

1887—1887(Hide quotations)

3. A device or instrument which gives a signal, etc., by sounding; also, the signal so given.

a1884   E. H. Knight Pract. Dict. Mech. Suppl. 832/1   Sounder, an alarm or call, made by closing an electric circuit.

1891   Pall Mall Gaz. 1 June 7/1   An electric sounder, too, is so arranged that it commences to ring if everything is correct, directly the gun is loaded and in the firing position.

>> One is reported as saying: "If I want to send a message, I use a sounder, or employ a boy to take it" (p.189). My question is: What, in this context, is or was a "sounder"

Telegraph.

https://en.m.wikipedia.org/wiki/Telegraph_sounder