Primordial black holes (PBHs) are widely considered as a hypothetical candidate of dark matter. However, the formation and astrophysical effects of PBHs still remain unclear. To gain the insights from the theoretical perspective, we recently proposed a novel mechanism of the sound speed resonance (SSR) cosmology that produces PBHs efficiently. I will briefly review the PBHs and the SSR mechanism and summarize what we have learned in this subject so far. I will also introduce the recent work on the application of the SSR mechanism into the stochastic gravitational waves which are expected to be a new probe for new physics in the early universe.