  Some Seyfert galaxies are detected in high-energy gamma rays, but the
mechanism and site of gamma-ray emission are unknown. Also, the origins of the
cosmic high-energy neutrino and MeV gamma-ray backgrounds have been veiled in
mystery since their discoveries. We propose emission from stellar-mass BHs
(sBHs) embedded in disks of active galactic nuclei (AGN) as their possible
sources. These sBHs are predicted to launch jets due to the Blandford-Znajek
mechanism, which can produce intense electromagnetic, neutrino, and cosmic-ray
emissions. We investigate whether these emissions can be the sources of cosmic
high-energy particles. We find that emission from internal shocks in the jets
can explain gamma rays from nearby radio-quiet Seyfert galaxies including
NGC1068, if the Lorentz factor of the jets ($\Gamma_{\rm j}$) is high. On the
other hand, for moderate $\Gamma_{\rm j}$, the emission can significantly
contribute to the background gamma-ray and neutrino intensities in the $\sim
{\rm MeV}$ and $\lesssim {\rm PeV}$ bands, respectively. Furthermore, for
moderate $\Gamma_{\rm j}$ with efficient amplification of the magnetic field
and cosmic-ray acceleration, the neutrino emission from NGC1068 and the
ultrahigh-energy cosmic rays can be explained. These results suggest that the
neutrino flux from NGC1068 as well as the background intensities of ${\rm MeV}$
gamma rays, neutrinos, and the ultrahigh-energy cosmic rays can be explained by
a unified model. Future MeV gamma-ray satellites will test our scenario for
neutrino emission.
