I think I know what you are trying to say. So I've given you a few more options to take a look at.
If you don't think it's right, let me know and I'll try to change it.
1) I thought his voice would sound different than what it is.
2) His voice sounds so different than what I expected.
3) I expected his voice to sound different.
4) I didn't think his voice would sound so different.
5) His voice does not match with the voice I expected him to have.
- For Teachers