I experienced this same problem for a while. If it is possible, you may have to record a few of your church services and let the musicians/singers hear the recordings. If they can hear how over powering they are, then hopfully they will adjust the sound levels. Also, if you are in charge, you can recommend more appropriate levels. What I tell musicians I work with is, "This is a ministry, if the message of the song cannot be heard, then all we have provided is "good music." Meaning, if the congregation/audiance can't hear what is being said, then all they will remember is the music and not the message.