  In spinful electronic systems, time-reversal symmetry makes that all Kramers
pairs at the time-reversal-invariant momenta are Weyl points (WPs) in chiral
crystals. Here, we find that such symmetry-enforced WPs can also emerge in
bosonic systems ($e.g.$ phonons and photons) due to nonsymmorphic symmetries.
We demonstrate that for some nonsymmorphic chiral space groups, several
high-symmetry $k$-points can host $only$ WPs in the phononic systems, dubbed
symmetry-enforced Weyl phonons (SEWPs). The SEWPs, enumerated in Table I, are
pinned at the boundary of the three-dimensional (3D) Brillouin zone (BZ) and
protected by nonsymmorphic crystal symmetries. By performing first-principles
calculations and symmetry analysis, we propose that as an example of SEWPs, the
two-fold degeneracies at P are monopole WPs in K$_2$Sn$_2$O$_3$ with space
group 199. The two WPs of the same chirality at two nonequivalent P points are
related by time-reversal symmetry. In particular, at $\sim 17.5$ THz, a spin-1
Weyl phonon is also found at H, since two Weyl phonons at P carrying a non-zero
net Chern number cannot exist alone in the 3D BZ. The significant separation
between P and H points makes the surface arcs long and clearly visible. Our
findings not only present an effective way to search for WPs in bosonic
systems, but also offer some promising candidates for studying monopole Weyl
and spin-1 Weyl phonons in realistic materials.
