'06 Volvo V50 drivers seat belt is jammed, there is no play in the belt and the ratchet locks immediatly no matter how gently it's pulled. Same problem with passenger side the day before but it righted itself. Any DIY advice?

I solved this on my 95 volvo 850 glt as follows:

pull the seatbelt pilar cover off from the door jamb. this runs vertically up the pilar between the front an back door (beside the front seats. Once opened loosen the belt by gently pulling the belt upward toward the roof. You'll have to do this sometimes in small increments. Once the seatbelt is pulled out all the way, spray small amount of lubricant into the joint where at the top of the belt spool. Also use a screw driver to push spool upward while pulling the belt. At first it's only going to release a little but eventually the lubricant will work its way into the seam at the top of the belt spool and loosen up the stuck inertia catch. Until it completely loosens you might have to remove the pilar cover again and work the seatbelt out (also use the screw driver technique described above) in order to use the seatbelt. When parking, I would relatch the seatbelt and pull the slack up around the back of the head rest. After a day or so it worked normally.

good luck and don't give up.