I believe I read this in the 90s as a hardback book while living in Kentucky. There was a family with at least two boys and one girl. They acquired some electronics equipment secondhand (I think from a friend who was getting rid of it) and they pull a prank where they expose the girl to the loudspeaker at a horribly high volume. It temporarily deafens the girl (she eventually gets her hearing back) but while deaf, she finds that she can hear thoughts.
There's another plotline that happens where the kids have to raise money to save something... a patch of land, I want to say. They scheme to make use of the girl's telepathy to cheat in a spelling contest by having someone else looking up the words and relaying the spelling to her, and they succeed in winning, getting just enough money to save the land, but didn't account for taxes. There's a brief discussion of donating money to charity to drop to a lower tax bracket, but they quickly realize that that would ultimately lead them to have less money than when they started. Fortunately, the child of a rich man has come up lost, and he's offering a reward, so they again use the girl's ability to read thoughts (I want to say that they unearth the speaker to boost her abilities, albeit at some degree of pain from the girl) and they find the child in an upended pipe on a construction site, possibly the site that they were trying to save.
The book had simple black-and-white line-drawings inside it for illustration. I remember one of the girl getting blasted by the loudspeaker, one of her brother sitting in a chair with a dictionary in his lap, and one where they were rescuing the child from a pipe. I think that it was one of the books where her gifts faded as her hearing returned to normal, returning her to normal, but it was hinted that they never quite went away entirely.