You can't tune the radio. You are supposed to tune the antenna.
I have a page for tuning your antenna. This can cause some distortion, but it shouldn't be that bad. Go here for how to tune your antenna.
also the internal speakers aren't that great. Try an external. I can't just use an internal because an external is so much better.
Something else to consider. There is a lot of skip right now. Skip is when the ionosphere becomes charge by solar flairs, and causes radio signals to skip across it, making them travel hundreds and even thousands of miles. There are some cb operators out there who run a lot of power. 10k watts and up, who just talk skip all day. A lot of those guys sound extremely distorted and over modulated. You may be hearing these guys out there. Its their radio's not your receive.
Also. People who are close to you, may over load the front of the radio in the receive section, which will make it sound distorted.