Music is the art of sound. It is a betrayal to the art and to the audience if the directors, producers and performers overlook this very basic criteria for a good performance, ACOUSTICS. The proper arrangement of voices and instruments can try to correct the lack of good acoustics or microphones. Consider also the arrangement of the song or music. That's why, among professionals, it is a common practice to rearrange the placement of choir members or instruments depending on the music performed. If you are observant the arrangement of the say, string sections of the orchestra changes depending on the repertoire.
Furthermore, it should be a requirement that a qualified engineer --- or at least the director himself/herself --- will check the actual sounds during the rehearsals and should bear in mind that during the actual performance the size of the crowd could affect the sound of the performance. I have attended some concerts/recitals, even by professional performers, that seem to neglect this. One of that is the string ensemble playing with Martha Argerich in Kitakyusyu, Japan. The producers just wanted to get more money with less performers.
There was also a performance where the choir is standing in front of the orchestra and there were two conductors -- one for the orchestra and one for the choir. Of course, the orchestra conductor couldn't see the choir conductor nor the choir could see the orchestra conductor. The music was a simple arrangement of a hymn but it was a mess. Meanwhile, the auditorium had a perfect acoustics but they used microphone which picked up the sounds of the alto and totally drowned the small symphony orchestra. Why would they do it? Only heaven knows.