If I never spoke up, never challenged my co-workers/managers and only did as I was told, I'm sure my experience at {company.name} would have been considerably better, but that just isn't the way I'm wired.
I now fully understand why my female co-workers often prefer to just stay quiet. After all, why would they risk their jobs to speak up about problems no one in a position of power is willing to accept the existence of?
It would be unreasonable to demand that they speak up, when doing so will all but guarantee their contract not getting renewed, being labelled as someone who's "difficult to work with" or have their career prospects damaged in some other way.
So please, the next time you hear a female co-worker speak up or complain; support her and remember that it's rarely a single occurrence, even if you never saw it happen, or only saw the one instance of it.