el son: This isn't related to the son the verb, but is a word meaning "sound." In everyday speech, you're far more likely to heard the word sonido.
la hueste: This uncommon word has the same meaning as the English cognate "host" in the context of this song. In current usage, las huestes is a bit more common as a way of saying "the army forces."
buena voluntad: Goodwill.
os dará: Os is an object pronoun meaning "you (plural)" that you'll hear mostly in Spain, very little in Latin America, although it is still commonly used in Latin America for liturgical use. So "salvación Dios os dará" means "God will give you salvation."
cante: Cante here is a subjunctive form of cantar, to sing. Cante hoy cada nación could be translated as "may each nation sing."
estas nuevas: Nuevas is one way of saying "news," so estas nuevas would be "this news." The word is plural even though it translates as singular in Spanish.
den: This is a plural command or plural present subjunctive form of dar, to give.
estas nuevas todos den: This sentence uses an inverted word order, which is fairly common in song lyrics and poetry. This sentence could be translated as "may all give the good news."
Belén: The Spanish name for Bethlehem. It is not uncommon for cities, especially those known centuries ago, to have different names in different languages.
salve: In this song, salve is an interjection of greeting, meaning something like "Hail!" in English.
Redención traído has: Another case of inverted word order. The typical structure would be "Has traído redención," "you have brought redemption." Note that this verse is sung to the savior rather than about the savior as in the English version of the hymn.
ala: An ala is a wing, as of a bird. This is a metaphorical usage here; "en tus alas la salud" could very loosely be translated as "with healing on your wings."
trono: Throne.
has bajado: You have come down. Bajado here is an example of a past participle.
la muerte conquistado: Another inverted word order. In normal speech, "has conquistado la muerte" would be more common for "you have conquered death." Conquistado here is an example of a past participle.
ser: Here, ser is functioning as a noun meaning "being" rather than a verb meaning "to be." In Spanish, most can function as nouns.
nacimiento: Birth. Nacimiento is a noun form of nacer, to be born.
