what do the words "mounting evidence" suggest about the author's intended meaning in paragraph 3 of this passage? Music and the Brain

When something "adds to mounting evidence" it means that it is growing rapidly, increasing, or the evidence that already exists is extensive and this new piece of information is added to that already established evidence pile. It supports it further.

In paragraph three of "Music and the Brain," the author writes that "Scientists believe there is mounting evidence to show that early musical training can improve brain function." The use of mounting evidence is used to show that there is a lot of evidence to show that when someone learns to play an instrument or sing at an early age will have greater brain function. This evidence is noted earlier in paragraph three.