Empirical evidence suggests that, for each positive integer $n$, the following equality holds:   
\begin{equation*}
\prod_{s=1}^{2n}\sum_{k=1}^{2n}(-i)^k\sin\frac{sk\pi}{2n+1}=(-1)^n\frac{2n+1}{2^n},
\end{equation*}
where $i=\sqrt{-1}$.

Is it a known equality?  If it is true, would you please give me some insights on how to derive this equality?