You can translate tone of voice as 语气 instead of using two '的'.
我们经常拒绝接受某个想法,仅仅是因为用于表述它的语气不够和善。
不同情 means not feeling sorry about something.
Here I feel unsympathetic means the tone of voice offends people, which is not nice or kind, that's why they refuse to accept it.
Maybe 不和善 is not the best answer.