What is opposite of soft voice?

The opposite of a soft voice depends on what you're trying to express:

If you're talking about volume:

* Loud voice

* Booming voice

* Shrill voice

* Powerful voice

If you're talking about tone:

* Harsh voice

* Rough voice

* Strident voice

* Aggressive voice

* Authoritative voice

If you're talking about the overall impression:

* Strong voice

* Assertive voice

* Commanding voice

* Forceful voice

The best opposite will depend on the specific context.