How loud is the first alert CO and smoke alarm?

Like anything else, that would depend upon where you measure the sound, among other variables. Many devices are calibrated according to how "loud" it is when measured 3 meters (10 feet) away, with standard temperature and pressure, in ordinary air.

The UL standard for CO and residential smoke alarms is a minimum of 85 decibels at 10 feet. If they are installed in a hallway, with carpeting on the floors, bedroom doors closed, and ordinary air seal around the door (i.e., not thoroughly insulated), this may not be enough to waken someone inside the room, in which case an additional alarm (or other alarm annunciator) should be installed inside the bedroom.