  We investigate the possibility of having massive compact stars,
$(M>2M_{\odot})$, and at the same time fulfilling the theoretical bounds on the
speed of sound $c_s$ in dense matter suggesting that the conformal limit of
$c_s^2=1/3$ is approached from below as the density increases. This is possible
if two families of stars exist: hadronic stars and quark stars, the latter
being entirely composed by quark matter. By using astrophysical data on
electromagnetic and gravitational waves signals of a few sources interpreted as
quark stars, we show, within a Bayesian analysis framework, that the posterior
distribution of $c_s^2$ is peaked around 0.3, and the maximum mass of the most
probable equation of state is $\sim 2.13 M_{\odot}$. We finally discuss also
the possibility that the maximum mass is larger than $2.6M_{\odot}$ as it could
be the case if the secondary component of GW190814 is a compact star and not a
black hole.
