DISCLAIMER: I am not a professional musician, but I've received formal Music education and can play several instruments.
DISCLAIMER 2: I've edited my answer, as it initially discarded "registración" as wrong. It is not.
I've never ever heard the word "registración" used in Music; I don't think it even exists in Spanish. Well, I definitely have, now.
In Spanish, registro is the most common word to refer to the pitch and timbre used when playing an instrument (e.g. "registro agudo" for a high pitch). It has some other musical meanings as well:
registro
- m. Pieza movible del órgano, próxima a los teclados, por medio de la cual se modifica el timbre o la intensidad de los sonidos.
- m. Cada género de voces del órgano; p. ej., flautado mayor, menor, clarines, etc.
- m. Cada una de las tres grandes partes en que puede dividirse la escala musical. La escala musical consta de tres registros: grave, medio y agudo.
- m. Parte de la escala musical que se corresponde con la voz humana.
- m. En el clave, piano, etc., mecanismo que sirve para esforzar o apagar los sonidos.
As you can see, one of them (16) refers literally to "each of the voices of the organ; e.g. principal, flute, clarion, etc.", which is pretty close to what you mean. It also can mean (15) each of the knobs used to achieve these sounds.
Both of these, the voices and its controls, are usually called stops in English but, as you said in comments, the voices can also be called registers which is closer to Spanish.
But! You don't want to translate "register". You want to translate "registration", meaning the combination of stops selected for a given piece. And you are asking if registración is the right term. Is it?
The word registración is not found in the DLE, which is the "official" Spanish dictionary; and, having such an specific meaning, it cannot be counted among the myriad of words that are not included "because their meaning is easily understood".
Maybe because of this, "registration" is sometimes translated as registro too:
Registro es el otro nombre que designa a los juegos del órgano, por lo tanto, registrar es el arte de combinar los registros para alcanzar el sonido deseado. Registro también significa la combinación de juegos que se hayan escogido o especificado, tal como en “emplee un registro tenue” o un “registro a pleno”.
[emphasis mine]
This is OK, the meaning of "registro" in the last two examples is perfectly understood.
However, after careful search, it is indeed true that the word registración is used today in Spanish by specialized blogs and authors.
Maybe these are all people that faced this very same problem and decided to go with registración in their translation?
A Google Books search for "registración" órgano shows that original works in Spanish using the term go back to 1965 only, with a 35-years prior translation of Hugo Riemann's work being the oldest result. Meanwhile, a search for "registro" órgano returns some results from the 19th century which talk about registration without actually using the word registración.
Still, registración is the word of choice in the official curriculum for Organ Studies in conservatories:
Órgano
GRADO MEDIO
Objetivos
La enseñanza de órgano en el grado medio tendrá como objetivo contribuir a desarrollar en los alumnos las capacidades siguientes:
[...]
d) Conocer y utilizar en los distintos tipos de órgano la registración en función de la época y estilo de la música destinada a ellos.
You can't get much more official than that!
All in all, I think the translation given by Google Translate is (for once) just fine:
I played it choosing a soft registration of Principal 8’ and Flute 8’ with the hope to bring peace to each one that listens to it.
La interpreté eligiendo un registro suave de Principal 8' y Flauta 8' con la esperanza de traer paz a cada una de las personas que la escuchen.
Fine as in, people will understand you just right.
But if you want to be more precise, and/or your translation is aimed at people that know their way with a pipe organ, then registración is probably the right choice, as its use seems to be widespread among organ players and officially recognized — even if not by the RAE.
TL;DR: You can use registro to avoid using a word that most lay people would label as incorrect, but registración is the most exact translation and is currently used among organ professionals and scholars.
