So I was looking online to see how many of the cast were fluent before the show and I found this great Slate article which includes how Katie Leclerc puts on a deaf accent to play Daphne, one of the titular switched daughters and one of the Deaf students. Leclerc learned ASL to fulfill a high school foreign language requirement, prior to learning she herself had Ménière's disease, which exhibits itself in fluctuating hearing loss due to fluid in the inner ear.
Leclerc thinks of Daphne’s speaking voice as being like any other accent, but rather than working with a dialect coach, she researched it with the help of her sister, an ASL teacher who also has Ménière’s. “She and I sat down, and we pulled out an audiogram, and we mapped out Daphne’s specific hearing loss and chose sounds that she could say and couldn’t say based on her exact hearing loss,” Leclerc told me. “It’s a very specific choice. From there, I just made my family crazy and spoke with the accent for two months until I felt I could do it.” When the show goes on hiatus, and she goes months without using Daphne’s voice, Leclerc pulls out the audiogram again and studies it until the accent becomes second nature.
I highly recommend you read the whole article.