Paul,

My understanding is that officers of light infantry battalions used whistles to convey commands in the field.

According to Windrow and Embleton, a normally reliable source, light infantry officers were issued with and used whistles to communicate in the field.
See plate 43: company officer 52nd foot, 1810.
Military Dress of the Peninsular War, 1808-1814, Windrow, M; Embleton, G. A. (Ian Allan 1974)
See also the first colour plate, not numbered, in Fetcher, I, Crauford's Light Division, Spelmount, 1991)

Best wishes
A