  The weak leptonic decays $\pi^\pm\to\mu^\pm\nu_{\mu}(\bar{\nu}_{\mu}),
\pi^\pm\to e^\pm\nu_e(\bar{\nu}_{e}),
K^\pm\to\mu^\pm\nu_{\mu}(\bar{\nu}_{\mu})$ and $K^\pm\to e^\pm
\nu_e(\bar{\nu}_{e})$ are investigated within the framework of the SU(3)
Nambu-Jona-Lasinio model, in the vacuum and in quark matter in $\beta$
equilibrium. It is found that, at low densities, the decay rates of the charge
multiplets, that are decoupled in this kind of flavor asymmetric matter, are
affected differently. As the density increases, these decay rates become closer
and show a tendency to vanish. This shows that weak interactions are strongly
influenced by the medium as a consequence of restoration of chiral symmetry.
