Fidelity Municipal Bonds Why Can A Municipal Bond Fund Go Down In Value?

Why can a municipal bond fund go down in value? - fidelity municipal bonds

If interest rates do not change, why create a municipal bond funds, value-added? The bonds are the interest earned on the funds held. It is money, but must be less than the accrued interest, and why there is a decline in value added and how long would it take to believe that not recover the interest rates rise?

Vi Fidelity Massachusetts Municipal Income (FDMMX) lose the value of a few percent over the last month and since interest rates unchanged did not understand why.