his voice sounded strange and i feel ___that there must be something wrong with him

certain是形容词，不能形容动词
形容动词需要用副词，所以是surely

其实也可以用certain啦，只是surely在这里听来语感更好。还有就是，人们一般都会说，i am certain about smth. 而不说，i feel certain about smth.

certain 很奇怪

certain = sure
Certainly = surely

But you can say "I feel certainly that there must be something wrong with him."