Yes, it is the House of God, and extended rehearsals and soundchecks immediately prior to Mass are gauche and impolitic for those who appreciate the transcendent silence of the inside of a church with the Blessed Sacrament in the tabernacle. But you have to understand that the technology of microphones and PA systems absolutely require that sound be tested and checked prior to use. Clearly this soundcheck should be performed as early as possible, before too many people are gathered inside, but it is technically necessary for the adequate performance of the Mass.
I have been in both extremes. In my visiting parish, my mother is beginning to get annoyed with the constant testing of mics and banging around of equipment immediately prior to Mass. This is because the choir rehearses in a meeting room and rushes into the choir loft mere minutes before Mass begins, and also one of the members is slightly disabled and has trouble walking, drops things, etc. On the other end of the spectrum is my home choir, where we sit respectfully silent until the moment Mass begins when I turn on all the microphones at full volume. This is risky business! Without testing them, we don’t know if we will get feedback, static, no sound at all, or some other sound disaster. Anyone could be tweaking the knobs in the week intervening between weekend Masses. Sometimes I have time to do a quick visual survey of the knobs to see if they are right, but more often than not I only check them after something is known to have gone wrong.
I agree that respect is due the Blessed Sacrament and others who are praying before and after Mass. But with technology comes the responsibility of ensuring that it is functioning properly, this needs to be done tactfully and discreetly so that others are not offended.