Bankruptcy Law Questions? Ask a Bankruptcy Lawyer.
It's great that you got your principal back! Most of these things don't turn out that well.
Because the firm has filed Chapter 7, they're completely liquidating all assets to pay creditors what they can - if there is any money available to pay anyone. Also, if your loan was not secured by collateral, you're an unsecured creditor - which puts you at the bottom of the totem poll.
That said, it is worth filing a proof of claim with the court. You can do this yourself or have an attorney do it for you. Here's the form: http://www.uscourts.gov/uscourts/RulesAndPolicies/rules/BK_Forms_Official_2010/B_010_0410.pdf
Other than filing a proof of claim, there's not much else you can do but sit tight and see if you get anything from the trustee. Chances are, no check will be making its way to you for the interest due.