Voice is a grammatical term that describes the relationship between a subject and a verb. There are two main voices:

  1. Active voice - John golpeó la pelota. (John kicked the ball.)
  2. Passive voice - La pelota fue golpeada por John. (The ball was kicked by John.)

The active voice is the most common: it indicates that the subject of the verb performs that action.

Mi amigo vendió la casa - (My friend sold the house.)
This verb is active - the subject (mi amigo) performed the action of selling la casa (the direct object).

In the passive voice, the subject of the verb undergoes the action being performed.

La casa fue vendida por mi amigo. (The house was sold by my friend.)
This verb is passive - the subject (la casa) underwent the action of selling and mi amigo was the agent.The emphasis changes from mi amigo to la casa. This type of passive voice is quite formal in Spanish and is not as used as it is in English.

There is a tendency to use an alternative which is called the Reflexive passive (La pasiva refleja) in which the invariable reflexive pronoun "se" is placed in front of the conjugated main verb. La casa se vendió(The house was sold). It is only possible to form a reflexive passive sentence with transitive verbs.

The passive is also formed with verb estar. In this case, it reflects a result after something has happened.La casa está vendida(The house is sold).  Here, as well as in the other passive voice type with ser,  the participle will be variable, taking the feminine/masculine/singular or plural form, agreeing with the subject.

La casa está vendida.

Las casas están vendidas.

Voice exists in all tenses:

Active Passive Reflexive passive
Los niños leen el libro.
The children read the book.
El libro es leído por los niños.
The book is read by the children.
Los libros se leen.
The books are/are being read.
El alcalde inaugurará la nueva fábrica.
The mayor will open the new factory.
La nueva fábrica será inaugurada.
The new factory will be opened.
La nueva fábrica se inaugurará.
The new factory will be opened.
El fontanero arregló el grifo. 
The plumber fixed the tap.

El grifo fue arreglado.

The tap was fixed.

El grifo se arregló.

The tap was fixed.

I'll be right with you...