My downstairs bathroom has a bar of deodorant soap (something like Irish Spring) that looks for all the world like it has bites out of it. I do have evidence of mice in a storage room but would never have thought they would eat soap.
Would they, could they, in a tub?
Mice are omnivores to the extreme. They will try to eat anything and everything.
That’s not to say it won’t kill them.
yes they will.
Did they say naughty words in front of their mommies?
At my moms I found a dead mouse near the wash basin and soap with obvious bite marks in it.
Mice used to get into my locker at my barn, and once ate a heavy duty plastic bag holding rubber bands – they left the leather goods alone. WTF?
It actually shouldn’t be too surprising that mice will eat soap when you consider that soap is basically a big chunk of chemically treated fat.
Traditional soap is. The fat is probably the most expensive ingredient in soap, though, so I wouldn’t bet any commercial soap has enough fat in a bar to provide even a mouse with a meal; also, not all fats are digestible, even by mice.
That said, mice will chew on anything at least once, more than once if it doesn’t kill them the first time. If whatever they bite isn’t edible, maybe it’ll be hard enough to keep their incisors worn down, or soft enough to be shredded for a nest lining.
I read this title as “Will Mice eat Soup” 
I wonder if a mouse would eat “mice soup”
I dunno about mice, but some marsupials will. I once had a wallaby crawl in between the fly and the tent when I was camping. There was a howling gale and he was comfortable there. I was too tired to stay up all night chasing him away. He chewed a hole in my tent, reached over my box of food (which has yummy peanuts on top) and took off with my soap. I found my soap with toothmarks in it about 20 metres away the following morning. And a hole in my tent.