Development of the contemporary predominantly visual sense of 'scout' dovetails with the parallel development of another term, 'watch'. The earliest uses of 'scout' were in the compound term, 'scout-watch':
The compound scout-watch n. appears in our quots. much earlier than the simple word.
1. The action of spying out or watching in order to gain information ....
4. a. One who keeps watch upon the actions of another; a watchman.
["scout, n.4". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/173223 (accessed January 19, 2016).]
Note that the noun 'scout' (in sense 4, above) is given as the etymological source of the verb 'scout' in OED Online:
Etymology: < scout n.4
["scout, v.1". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/173226 (accessed January 19, 2016).]
The earliest quotes attesting the compound noun 'scout-watch' are from sometime before 1400 through the 1500s:
a1400 Morte Arth. 2468 Skayres thaire skottefers, and theire skowtte-waches.
c1400 (▸?c1380) Cleanness l. 838 In grete flokkez of folk, þay fallen to his ȝatez, As a scowte-wach scarred, so þe asscry rysed.
1442 Beckington's Jrnl. (1828) 97 Whan they were approched nigh the same towne there comme upon theym the skoulk wache, and there a showte was made of St. George d'Angleterre.
1563 Burnynge Paules Church sig. Bv, The word Episcopus is Greke, and signifies a Scoutwatche, an ouerloker or Spie.
["† ˈscout-watch, n.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/173237 (accessed January 19, 2016).]
The earliest quotes for 'scout' without compounding appear in the mid-to-late 1500s for both sense 1 and sense 4 (as shown above):
1553 J. Brende tr. Q. Curtius Rufus Hist. iv. f. 34, But those that discouered for the Percians, were but a thousand horsemen, which keping the scoute a farre of, semed to the Macedons to be a great army.
(from sense 1)
1585 A. Munday tr. L. Pasqualigo Fedele & Fortunio sig. C3v, As close as I can, in this place I wil stand. Unseen vnto any, yet vewing of all: A prety scowte set to take a knaue in a pit-fall.
(from sense 4)
The noun 'watch' did not at the time of the development from 'scout-watch' to 'scout' involve the primarily visual denotations of the contemporary verb 'watch'. Rather the noun denoted something more akin to 'wakefulness' and 'vigilance',
I. Wakefulness, vigil.
†1. a. The state of being awake; voluntary or involuntary going without sleep; wakefulness. Obs.
[quotes from around 1000 to 1631]
II. Action of watching or observing.
6. a. The action or an act of watching or observing with continuous attention; a continued look-out, as of a sentinel or guard.
[quotes from 1377 to 1901]
["watch, n.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/226075 (accessed January 19, 2016). Emphasis in 6.a. mine.]
Toward the end of the period during which the noun sense of 'watch' developed a visual sense ('look-out'), in the late 1400s, the verb sense of 'watch' developed its contemporary primary visual sense:
4. a. To be on the look out; to keep a person or thing in sight, so as to be aware of any movement or change.
[quotes from 1487 to 1860]
["watch, v.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/226076 (accessed January 19, 2016).]
All the foregoing tracking and tracing of the intertwined senses and development of 'scout-watch', 'watch' and 'scout' is to the point, with reference to your question, that while the 'scout' in 'scout-watch' is etymologically related to an Old French word denoting 'the action of listening', on adoption in English for the compound 'scout-watch' the sense was originally more at 'sentinel' or 'guard'. That is, 'scout' in the sense of listener was adopted for use in a term that combined the sense of a listener with the sense of one who is vigilant, stays awake to observe.
As the compound 'scout-watch' developed into 'scout' (1400s-1500s), the weak remaining connotation of 'listener' was weakened further if not lost altogether, subsumed into the more general sense of 'an observer for the purposes of guarding'.
Then, after 'watch' developed its contemporary primarily visual sense (late 1400s-1500s), it became detached from 'scout-watch', but left behind the primarily visual senses of the contemporary use of 'scout', with only the vestigal sense of 'listener' implied by 'vigilance'.