Dear Vale,
It's peculiar to see that in this 'villancico', the words of the title are not repeated in the lyrics. The version I Know, popular here in the United States says:
"Angels we have heard on high
Sweetly singing o'er the plains
And the mountains in reply
Echoing their joyous strains..."
I suppose you know it. There are already a couple translations in LT.
Anyway, I have a couple suggestions for you, on your translation:
1. Title- "hemos oído en alto" --> "en lo alto" (en las alturas), as a noun, not an adjective. And I wouldn't use "que", -->"Angeles hemos oído..."
2. second verse "alad", wouldn't it be "alzad"?
3. Second stanza- "doblando delante..." --> "inclinándose ante el altar".
Second verse, "Mirando mucho tiempo en esperanza y miedo', aside from long, this verse is not very clear to me: "esperando/velando CON esperanza..." (?)
I prefer "de pronto" to "de repente".
4. Next stanza - "Reunirá todas las naciones a Él" --> "a Sí (mismo) (?)
"saludándole" --> "adorándole" (?)
Difícil estilo, Vale.
Te deseo luminosas navidades en un Estocolmo sin sol.
He dejado sin traducir las palabras latinas.
Gloria, in excelsis Deo – significa: Gloria a Dios en las alturas