I have never heard the concept that lead pellets won’t break the sound barrier.
Pretty sure all mine, of different weights, are lead.
After reading reviews I bought the heaviest rounds I could find.
I have about two dozen pellet rifles including a few gammos. Several of them will break the sound barrier with pba or alloy pellets, none of them get close to the sound barrier with lead pellets. I would recommend a gammo big cat whisper, it is extremely quiet with led ammo right out the box. With pba ammo it sounds like a 22lr. The gammo big cat is also very inexpensive, I think I paid about $130 for it. Fortunately for me my neighbors would not care if I were shooting in my yard, and I see them plinking some times. Have fun shooting.