Synesthesia is a phenomenon where one kind of stimulus leads to another. Synesthetic people can perceive sounds as colors or feel that numbers have a certain texture or personality. It has absolutely nothing to do with perceiving people's state of mind as colors, which is quite impossible in the real world.
Actually, seeing auras around people can be a type of synesthesia. It could be caused by the brain interpreting emotional cues as colors (more on the ideiosthesia side of synesthesia) or interpreting pheromones and other variations of scent cues the body gives off depending on the emotions one is experiencing as color. There's also two flavors of synesthesia, projective and associative. One with the first type would see the color around someone while the second type would simply associate the color of their mood with the person.
Outside of that, Canaan does make repeated mention of "scents", so it could be that she has a smells -> colors variety... and has a nose like a bloodhound.
Also, synesthesia doesn't work the way it is shown in episode 10! It is a process inside our brains, so if some device (see below) does cancel out all noise, it won't reach the synesthesiac person's ears, ergo, there is no way it can be converted into any other sense and do any damage, if it's Hakko's voice.
The explaination for all of this lies in the fact that Canaan's synesthesia is ultimately born from the Ua virus. Considering the other "Borners" there is not real reason why it would have to follow the same rules.
Active noise control, which cancels out all sounds, is a realistic notion, except that to cancel sounds in the entire facility, as shown in episode 10, it'd have to consist of thousand of sound detectors and hundreds of anti-noise generators scattered all over the place. And a huge mainframe to process the noise data in real time.