True (though stealth bombers are subsonic, just). What about headings that are not straight at you though? May still have value detecting things passing by?
When I was at military I attended a few exercises where jets were involved. They don’t go supersonic but pretty close to it so usually once you start hearing something it’s almost over. It’s actually quite terrifying to have a jet coming at you at 800 km/h at low altitude. Very surprising, then very loud and incredibly fast. You also hear passing by jets only once they are long gone. I assume radar is just much better than sound.