This is the strangest condition I have ever heard of. It is called synesthesia and takes many forms. I hate to simply post a link but it is too far reaching to summarize. This just fascinates me as I try to imagine it happening.
Dennis
As the article indicates, there are many forms of synesthesia, and I believe that it is, if not common, more prevalent than a person may think. I experience what I believe are mid-level mirror-touch synesthesia and low-level spatial sequence synesthesia. With the mirror-touch, when I see someone being slapped, for example, I feel the sensation of being slapped, but not the pain associated with it. It’s a strange thing that happens involuntarily, reflexively. But it doesn’t occur every time I see such an action, which is why I say I think it’s mid-level. In the reading I’ve done on synesthesia, I’ve come away thinking that it’s a term used to describe a great many perceptions and may even be too broad.
Se7en is La7ender.
I used to belong to a message board for synesthetes. Sadly defunct now, but I have many fond memories of the place.
In Grade 2, I wondered why my teacher never mentioned that 2 was red and 5 was blue.
Cuisennaire, right?
Two was red
Five was blue
Now you’re hearing Barney, too.
Because 5 is yellow-green (same as E minor). You’re right about 2, though.
No, 6 is yellow-green. 7 is a weak green. 8 is purple.
a little help here? 
Nope, that’s not it.
When I thought about the digit “2”, it was a bright red in my mind’s eye. Not the word “two”, just the digit. The digit “5” was a bright blue.
Never seen those sticks before.
I don’t know whether o laugh or hunt you down to strangle you with your mouse cord!
Ear worm alert!
(All of the above is a jest, of course. I would never try to strangle a fellow doper, even one who inserts Barney ear worms in my head. No matter who much said Doper deserved it. 
)
Which begs the question, do numbers/colors/sounds translate the same for different people.
Not the same thing as the numbers, but there are high intensity street lights of some kind with a yellowish tint and I can taste the light.
Oh damn, those colors are ALL WRONG!!!
I experienced it much more strongly when I was younger. But I still see numbers as colors. As a purely visual thinker I think this helps me with math.
When I was younger I had the same thing with letters. I was something of a spelling savant, which I’m sure was connected.
I don’t know if it’s related, but all my life, whenever I look at any inanimate object–literally anything; a leaf, a sock, a chair–I immediately orient it in my mind to a human face or form. Chairs have eyebrows, and hands on their knees. A maple leaf has arms and legs. A jar has shoulders and a face. I’m sure this has contributed to my life as an artist; everything has a personality and a visual balance
Drop some acid and you might well see musical sounds transmuted into colors pouring out of your speakers.*
*or so I am told. 