"how Baudelaire was able to apply to the sound of the trumpet the epithet 'delicious'"

Baudelaire by Gustave Courbet
Public DomainBaudelaire by Gustave Courbet
Charles Baudelaire (1821-1867) was a French decadent poet, known for his collection Les Fleurs du Mal (The Flowers of Evil). The 'delicious' sound of the trumpet comes from the poem L'imprévu (The Unforeseen):

Le son de la trompette est si délicieux,

Dans ces soirs solennels de célestes vendanges,

Qu'il s'infiltre comme une extase dans tous ceux

Dont elle chante les louanges.

 

And so deliciously that trumpet blows

On evenings of celestial harvestings,

It makes a rapture in the hearts of those

Whose love and praise it sings.

(Translated by Roy Campbell, Poems of Baudelaire, New York: Pantheon Books, 1952)

For the complete poem, plus English translations, see here.