I had to dig up my old neuroscience notes. After my notes this is called "Synesthesia". "[It] is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to involuntary experiences in a second sensory or cognitive pathway" (Wikipedia).
After my notes only 1 in 25'000 people (Cytowic, 1988) have that. And after Wikipedia the effect vary strongly.
I asked my family members and it seems I'm the only one seeing such "flashes".