You'll need to contact the person at your bank who handles your case. If you don't know who that is, then you'll need to contact someone in your lender's loss mitigation department to check on the status of your short sale.
No, the bank isn't the investor. Banks make money by lending out other people's money (ie the deposit base [savings and checking accounts, CDs, etc], stocks, bonds, etc) at rates higher than they pay the other people to use their money. Stated another way, banks make money via positive leverage. Before the bank could lend you the money for your home, the bank had to find an investor willing to lend the money to the bank, that the bank lent to you. Investors do this, so they can earn money from the cash-flow that your loan generates. However, since you've entered a short-sale negotiation, the bank essentially has to approach its creditor (the investor) essentially to do the same thing. Ultimately, this process can take quite a while, because many lenders sold their loans to other entities (companies or individuals) which also in turn sold the loans to yet other entities. The point is someone has to trace that entire chain until they find the one who actually owns the mortgage now.