You might sometimes see a yellow zone sign "SIGN LANGUAGE SPOKEN HERE" at some places, such as school for the deaf, organizations of the deaf, offices of the ASL instructors, and so on. It is a friendly warning sign to indicate that the area is to use ASL only or where everyone uses ASL.
No voice policy is a standard policy in ASL instruction. ASL is taught entirely in classes with a "no voice" policy. From the time you arrive in class, all conversation should be in ASL until you leave at the end of class.
The classroom is a speech-free zone for two major reasons: learning environment and cultural respect. Not only it is a classroom policy, it is also a socio-cultural norm and custom in the Deaf world. Learning a language is not without learning its culture, inseparable.
A student's learning ability is greatly enhanced by this no voice environment. Full immersion helps develop better receptive and expressive skills. Using voice may distract or interfere other students' learning process.
In the early stage of learning ASL (e.g. level 100), "no voice" policy is crucial. Some students may think it is easier to or may be tempted to learn ASL vocabulary by using vocal English. Easy start but bad start -- it doesn't help in the long run in receptive skill and language acquisition through eye. It's probably challenging for some of them in the beginning but a breakthrough will be rewarding in the long run. Those who choose the easier way in the beginning may face a more challenging receptive/expressive skills in the later stage. Patience and persistence will pay off.
Another reason is to maintain that ASL is a language of its own. Students are taught to think in ASL, not English while signing ASL. Using voice (English) while signing ASL may interfere with language development in vocabulary and grammar: incorrect uses of ASL vocabulary in different contexts (semantics), an incorrect grammatical structure and/or a limit of grammar skill.
The use of voice in Deaf space is a rude or offensive behaviour. It demonstrates an ignorance or disrespect for their culture as well as a painful reminder of hearing oppression as well as personal experience. It ties with a long history of oppression and audism that Deaf people endured for hundreds or thousands of years.
Classroom is a good place to train and learn to practice "no voice" custom that will be applied to a cultural behavior and norm in the ASL/Deaf world outside a classroom. Hearing people who know ASL usually talk in ASL in any ASL/Deaf space (e.g. Deaf club, party or event, deaf school, social gathering, etc.) or in front of any Deaf person to respect their culture and language.
Language and culture are inseparable, intergrated, and intertwined. Your learning involves your process of allyship. As ASL is the most valued identity of our culture, teaching you our ASL language is regarded a precious gift of from us.
Learning ASL in class is usually a fun and interesting experience. Students often have a positive learning experience, but also make sure the ASL (Deaf) instructors have a positive teaching experience also. Respect is a peaceful language across all languages and cultures.