  In \cite{St3} H. Stetk\ae r obtained the solutions of Van Vleck's functional
equation for the sine $$f(x\tau(y)z_0)-f(xyz_0) =2f(x)f(y),\; x,y\in G,$$ where
$G$ is a semigroup, $\tau$ is an involution of $G$ and $z_0$ is a fixed element
in the center of $G$. The purpose of this paper is to determine the
complex-valued solutions of the following extension of Van Vleck's functional
equation for the sine $$\mu(y)f(x\tau(y)z_0)-f(xyz_0) =2f(x)f(y), \;x,y\in G,$$
where $\mu$ : $G\longrightarrow \mathbb{C}$ is a multiplicative function such
that $\mu(x\tau(x))=1$ for all $x\in G$. Furthermore, we obtain the solutions
of a variant of Van Vleck's functional equation for the sine
$$\mu(y)f(\sigma(y)xz_0)-f(xyz_0) = 2f(x)f(y), \;x,y\in G$$ on monoids, and
where $\sigma$ is an automorphism involutive of $G$.
