How can you improve sound quality in an auditorium?

Have the auditorium's dimensions measured and have the facility acoustically treated. Be sure the speakers and amplifiers are power matched to achieve best overall sound with minimal clipping and distortion.

If cost is an issue and sound treatment isn't accessible, speaker placement is the next best thing. Depending of the speakers' angle of dispersion and how large the seating area is, direct the speakers in a way so that maximum coverage is achieved. Try not to overlap the the sound coverage when directing the speakers as this can cause "hot-spots" or areas may be louder to the listener. Be sure the speakers and amplifier are connected correctly (positive to positive and negative to negative). Having any of the poles reversed can cause phase cancellation resulting in most, if not all, of the frequency ranges cancelled out, creating a hollow and weak signal.

If an equalizer is available, use it to optimize the sound by boosting or cutting certain frequencies that may be lacking or that may cause unwanted feedback.