A sledgehammer-wielding robber smashed through a Bronx jewelry store’s front window, snatching $20,000 in chains and pendants, surveillance video obtained by The Post Wednesday shows. Screams can be heard on the video from inside M & NP Jewelers on White Plains Road during the smash and grab around 4 p.m. Sunday – before a roll-down gate nearly ensnares the culprit. A witness told the NYPD “three unknown individuals broke the store window with a sledgehammer” and “removed assorted jewelry,” police said in a statement. The video shows the robber who wielded the sledgehammer shoving the baubles into a bag held by a second man. It’s not clear what the third man was doing, cops said.