My Infiniti radio in my kia amanti ..the volume has quit but everything is working, what's wrong and how to fix it??

Not familiar with this particular radio but many radios in vehicles these days have a "mute" feature. This lets you easlily silence the radio without actually turning it off. The button for it will often be symbolled with the shape or a speaker with the circle with a slash line through it that stands for "No". Check and see if your's does. Maybe you activated it by accident somehow. When activated the radio will still do everything except play from the speakers.