Well, whether it's over a speaker grill or a piece of clothing, cloth is cloth. Use a lambert or other very-low-specular material, and find a high-res photo of the texture or create your own in Photoshop. You'll want colour and bump at least, maybe diffuse and specular if you're getting really fancy. Don't rely on Maya's cloth texture, which isn't really up to the task.
When I think of big speakers at a concert, I think of little bits of dust and fluff on them. You might attach some particles or something to achieve that effect, if you need more detail than could be achieved by just painting it in to the textures.