Islam does not take it good to sing salam after salah collectively in high tone. First, it is sheer display which ignorant people do to show others. Also singing salam like this is neither proved from the companions, nor tabi’in or Imams, this is the invention of ignorant innovators.
It is high ranked prayer to send durood and salam to the holy Prophet Muhammad (PBUH) but it should be recited individually in privacy and slowly and earnestly only for the pleasure of Allah. Recite it hundreds or thousands times, it is very good and rewardable.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband