The noise generated by the outdoor Pacific Amphitheatre in Costa Mesa continues to be a hotly debated issue in Orange County. Orange County Superior Court Judge Richard J. Beacom and representatives of both the pop-rock venue and community action groups began meeting Wednesday to determine whether the amphitheater must get their acts to turn down the volume. Richard Spix, an attorney representing Concerned Citizens of Costa Mesa Inc., said he is "not optimistic" about a settlement. Deborah Nesset, who represents the amphitheater's management, Ned West Inc., had no comment Thursday. If a compromise settlement is not reached, several court cases now pending could force the issue. Last May, the residents' group obtained a court order prohibiting arena noise from remaining at 80 decibels--about the sound of a vacuum cleaner--for more than five continuous minutes.