Scion said:
A glamor/figment can make sound 'seem' to disapear, but this would give no protection vs sonic attacks. Silence provides protection.
SRD:
Because figments and glamers (see below) are unreal, they cannot produce real effects the way that other types of illusions can. They cannot cause damage to objects or creatures, support weight, provide nutrition, or provide protection from the elements
Just because you dont know something is there does not mean it cannot hurt you.
That would be true, if what a glamer/figment did was make you think you heard or did not hear something. Since they are not mind-affecting, they do not do this. Illusions of this type arrange it so there actually is or is not a sound to be heard. It, unlike a phantasm, changes the objective state of the area, the changes being limited to changes of sound and appearance. What the illusion must do, then, is to actually change the volume level of sound in the area, either creating volume and pitch or dampening it.
As a sonic attack is supposedly a very loud and focused sound, it is affected by the glamer in exactly the same way as a more quiet sound is. The sound is suppressed inside the effect. No sound means no wave of force, because the wave of force is composed of sound. An illusion cannot create a sound loud enough or at a pitch at which it can do damage, because that is expressly forbidden. But preventing such damage by eliminating the sound that causes it is not forbidden, and this is exactly what Silence does. There are no other illusions, to my knowledge, that can eliminate a sound completely. Silence fills this role, and that's why it's the one that can provide protection. Any other illusions that completely dampen sound should also provide protection.
You'll notice that the "* Image" series of spells only create sensations. They do not remove them. You can use them to disguise, but not eliminate the prevailing sensory conditions. If you put an illusionary wall in front of something, you can't see through it, but likewise, if you sound an illusionary trumpet it might make it hard to hear footsteps approaching. In either case, figments can be disbelieved if interacted with, which negates their disguising effects.
Nothing real is created by the silence field. It is simply an area in which sound cannot happen. If an attack is composed of sound, it cannot happen in that area.