Yes, even if several people say the same thing simultaneously, with the exact same voice and no audible differences, we can still know that there are more than one person. This knowledge is possible due to two main factors: visual cues and spatial separation.
Visual Cues: When people speak, they also produce visible mouth movements, facial expressions, and body language that can be observed by others. Even if the sound is identical, these visual cues provide evidence that there are multiple individuals present. We can see multiple mouths moving, facial expressions, and body positions, which clearly indicate the presence of more than one person.
Spatial Separation: Sound waves travel through the air, and our ears are sensitive to the direction from which the sound is coming. When multiple people speak the same thing at the same time, sound waves emanate from different positions in space, creating a sense of spatial separation. Our brain processes this spatial information and recognizes that the sound is originating from different locations, further confirming the presence of multiple individuals.
These two factors work together to allow us to discern that there are multiple people speaking, even if the sound they produce is identical. Our brain combines the visual and auditory information, enabling us to differentiate between distinct individuals in our surroundings.
Additionally, even though the sound itself is identical, there are often subtle individual differences in how people produce sound, such as the slight variations in pitch, timbre, and articulation. These differences may not be apparent in a specific case of people intentionally trying to speak in an indistinguishable manner, but in general, they contribute to the uniqueness of each person's voice.