Sealing the gaps at the clapboards is generally a bad idea, the clapboards
are designed to allow the wall to breath and let moist air to pass through.
Sealing of the gaps may increase the chances for condensation. Damage
clapboards that allow water ingress should be repaired/replaced. As far as
the desire to modify the exterior wall system to increase the insulation,
add a vapor barrier and reduce sound transmition. This type of modification
should be carefully designed with atention paid to the buildings mechanical
systems, the complete wall, ceiling, floor, roof enclosure, venting of
attics and source moisture control from both exterior sources such as good
roof and ground water management as well as internal mosituree sources such
as kitechens, bathrooms, laundries, etc. Most of the post-construction
problems that I have seen occure involve the unsucessfull attempt to create
a complete moisture barrier at existing exterior enclosures.