You have to pay attention to detail and think about these things.
* You have two functions - one called isVowel and one called countVowels. In your main function, you say "There are " isVowel(sum) " vowels in this sentence". Does it make sense to output isVowel, or maybe it would be better to output the result of countVowels. Also, shouldn't you be counting the vowels in the string you read in - myString - instead of sum?
* Your isVowel function should probably just check a single character like it did originally. So instead of taking a string, it should probably take a char.
* To compare a char to a character literal, you would do something like this:
if (myChar == 'A' || myChar == 'B' || myChar == 'a' || myChar == 'b')
notice the use of ' instead of ", and notice that you have to use == each time. You should be able to adapt that example for use in your isVowel function.
* Your countVowels function takes a bool as you have it written. You should probably be counting the vowels in a string, so maybe you should change it to take a string as an argument instead.
* Your loop in countVowels doesn't make sense. You want to loop through every character in the string. The characters in the string go from index 0 to one less than the length, so you can go from index = 0 to index < myString.length().
* You want to have two separate variables in your countVowels function - one for looping through the string and one for counting the vowels.
See if you can put those changes in and good luck. Remember to think about what each thing is doing or is supposed to do and pay attention to each detail.