Explosion (Ex) As a standard action, a yrthak can fire its sonic lance at the ground, a large rock, a stone wall, or a similar nonmagical object within 60 feet to create an explosion. This attack deals 2d6 points of piercing damage to all within 10 feet of the effect's center.
What are "similar objects"? Objects that can shatter, like stone or glass? Or anything?
I'm designing a town that deals with yrthaks regularly, and I'm thinking about basing their limited architecture around avoiding yrthak explosions. If the "similar objects" are just any objects that aren't magic, though, I won't bother.
I'd say anything that can break into piercing objects. So wood would work, but tapestry wouldn't.
The sonic lance is a ranged (ray) attack that deals 8d6 sonic damage. It's an attack roll, so it can make critical hits.
Objects generally take half damage from energy based attacks, though in specific cases they can take more or less. Objects are not subject to critical hits.
So what this means is that you are dealing with a maximum of 24 damage to an object made of an appropriate material. If the material has hardness in excess of this, it is immune to damage from the yrthak and probably couldn't be used to cause an explosion. Of course, that doesn't exactly help - adamantine has a hardness of 20. Magically treated walls double the hardness, but cost 1,500gp per 10 foot segment. If the town is rich enough, it put up magically treated adamantine walls with 40 hardness.
More likely, though, is that a town in an area besieged by yrthaks would be located underground, where the yrthaks cannot find them.
Alternately, a series of permanencied silence spells would do the trick quite nicely - not only would it render the town immune to the yrthak attacks, it would also render it effectively invisible to them.
Or for a cheaper solution, all their houses are made of straw.
Exploding straw is likely much less dangerous than exploding wood, stone, whatever.