Culturally, mothers in China did not have to explain things to their children. They trained their children to carefully observe and imitate, to recognize authority, and to learn by inference. Mothers were generally silent in China. They existed in a high-context culture where understanding came from people who "knew" the same things and made the same "assumptions." In fact, it was considered an embarrassment to have to explain oneself as if the speaker somehow did not have the capabilities to adequately convey one's meaning without explanation.
In coming to America, a low-context culture where meaning comes from discussion and explanation, the Chinese mothers were at odds as to how to convey the knowledge their children needed to know. Ultimately, they adopted the "story-talk" technique which allowed them to shape their own personal "stories" of life, their "cautionary tales" into something strongly resembling folklore. This provided them with distance from their audience and allowed them also to adjust to a new culture and pass on information to their daughters in a socially-accepted way for a Chinese woman/mother.
Tan's fiction captures the experience of both first generation and second generation migrants to the USA. The stories that she tells of her first generation migrants captures the way that cultural norms and identity are ignored, not treated with respect and denigrated. All of which of course results in the metaphorical silencing of their identity.
The power of cultural tradition weighs heavily and cannot be shaken even when coming to a new country. In addition, many immigrant groups have faced silencing upon coming to America. In Joy Luck Club, while the women faced cultural silencing in their homeland and carried it with them, they faced further silencing of hostility and dislocation as "the other" in America.
Women are expected to defer to men, especially in public. In both cultures this is true, even though it is moreso in Chinese culture than in American culture. Women are expected to make sacrifices and to provide for family in addition to whatever else may be going on...jobs outside the home, educational pursuits, etc.
Are you asking metaphorically, as in they have no voice? They have no voice because they maintain the culture, even when they come to America, and because they are discriminated against in America. They are oppressed by men or the dominant culture, or both.
The universal condition of sexism is something that Tan would say causes both sets of women to be silent. Tan is making a very interesting argument within such a paradigm. Essentially, she argues that the condition of being a woman and dealing with the silencing of voice is one that cuts through cultural lines. The patriarchal condition of both China and America is one that relegates women and their needs to the peripheral. However, she is arguing that acknowledgement of this condition can help women to regain their voices, and, in turn, silence the social practices of silence.
For the mothers who were daughters in China, the social and traditions there moved women to the fringe of the social order. The daughters were silenced through social pressure and being told to capitulate to such a condition. Yet, Tan does not allow these daughters who will become mothers to remain silent. Instead, they have to actively break the mold and speak, sometimes scream, in order to be heard. When these daughters become mothers, they are silent in that they are immersed in a different culture with different expectations. In America, the freedom that they believe is a part of their identities and the identities of their children is what causes them to be initially silent. Yet, each mother in their own way recognizes that their daughters are being subjected to a more insidious form of sexism than what they experienced, which was far more overt. Yet, silencing voices is still silencing voices and each mother summons the courage to ensure that their daughter does not endure the same hardship that they have. It is here where mothers and daughters experience what it means to be silenced, but do not succumb to it. In finding their voices, both mothers and daughters no longer are silent.