Voice Acting is the action of a Scratcher recording their voice in a project and allowing another Scratcher to use it in their own project. This is a common practice on the Scratch website. Scratchers can either record their own voices onto the project or use voiceovers from other Scratchers, often through voice auditions.
Voice auditions are projects which Scratchers remix in order to audition for a certain part of a project. The Scratcher adds their voice recording to the remix, often saying a specific line given to them by the creator of the main project. Voice audition projects usually have a choice of several characters to audition for, giving a slide of information (generally including a picture and details about the character's story and voice) for each character. Depending on the creator the auditions may work on a first come first serve basis or judge on skill. It's also possible for some to cast people in different roles than they wanted, although the voice actors/actresses don't have to agree to it. 
Voice Acting Shops
- See also: Shop (Project Type)
A voice acting shop is where Scratchers can go to either get a voice actor to be in their animations or to volunteer as one and work for the shop. Generally, the shops also include descriptions of the voice actors with things like the gender or age of characters they generally voice. Shops are usually forums in the Scratch Discuss section Requests. There are several of these on the Scratch website.
Alternatively to this, you can make a new topic asking for voice acting.
Voice Actors are the people who record their voice and give it to others. Their voice can be used in projects such as animations and games, and it is normally given to the recipient through use of the backpack.
Voice Acting is useful for Scratchers who would like to add a wide variety of voices to their projects. Voice Acting is often used in animations or songs. (See: I'd Like to Teach the World to Code) It is also useful if a Scratcher does not want their voice to be online, does not have a microphone to record their voice or who need a voice very different from their own. Sometimes Voice Acting is needed in animated series.
- Voice acting on Wikipedia