Sonic, is fairly straightforward. It generally describes something which is travelling at the speed of sound, or 768 miles per hour, quite fast. Since you are asking in relation to a band however, it could be that they are simply using the word as a more interesting description of anything relating to sound itself.
Adding "Titan" in this context leads to several interesting intentions. Though it's origins are in classical mythology, it's meaning has expanded and have now been incorporated into the general lexion. In the general sense, anything could potentially be referred to as a titan if it is somehow many times larger in scale to the things around it. This could be in physical size, or importance. In a more modern sense, it is mostly used to recognize individuals who have shown themselves to be "giants" in their field.
So, sonic is in reference to great speed, as well as sound. Titan is in reference to great size. Because speed and size are often in conflict with each other, this conjures an interesting juxtaposition. Perhaps the bands in this genre, who are so often drawn to fantistic imagery, wanted to paint a picture of something awesome and difficult to imagine. Or perhaps they see themselves as titans of their realm. In the case of Sleep and Electric Wizard, this is arguably true. I been drinking.