How much weight can an upstair room can hold?

YOu can ask your local building official what your state building code is based on for a residential structure second floor. Also you can figure that a 200 lb amp = one fat-a$$ed American and draw your own conclusions. And remember that most building codes would be based on a load in terms of weight per square foot so a 200 lb amp is probably providing a similar load to a 200 lb person standing up.

Hevac1 is correct. one thing you can do is use some tape to mark the ends of the crack to see if it is elongating. Elizabeth is correct about storage of LPs, but 500 lbs of gear should not be a problem at all. Also consider where the gear is located. If its arrayed against a load bearing wall and spread out so its over several joists, then the load on any one joist is small and transferred to the vertical load-bearing studs and plates that are designed to carry much bigger loads. If you are really worried about this, call an architect or structural engineer. Would be well worth a couple of hours of their time (if they charge you at all; they may be able to answer this question very quickly in a brief phone call) considering the cost and hassle of selling all your gear.

Andrew- If the crack is in your garage ceiling and the garaage is unheated, its probably a thermal expansion/contraction thing, esp w a heated/cooled room above.