Someone with sound-color synesthesia sees colors and simple shapes whenever they hear a sound. The same sound always occurs with the same colors and shapes. Someone with taste-touch synesthesia feels touch sensations in their hands every time they taste something with their mouth. The same taste always occurs with the same feeling of touch in their hands. The particular connections between sound and color that one sound-color synesthete experiences typically differ from the connections experienced by another such synesthete. In this sense, the connections are an arbitrary convention. Now imagine a sound-color synesthete who no longer has sound experiences to acoustic stimuli, and instead has only their synesthetic color experiences. Then this synesthete would only experience as colors what the rest of us experience as sounds. In principle they could get all the acoustic information the rest of us get, only in a color format rather than a sound format.
Donald D. Hoffman's thought experiment is mind-blowing and triggers another question: What would thought synesthesia be like ? What if one's ideas could be felt with senses ? What if you could touch my happiness or smell my courage ?
What would it sound like ? Would an epiphany sound like Beethoven or Maroon ? Would intense focus feel like caressing a straight sleek surface ? Would passion march like a red orchestra weaving tulips ? Would fear look like the north pole, two bare feet on the water and a tam-tam playing under water ? And what would it smell like ? A leather scented perfume with burnt cocoa notes ?