An SR20 Instructor and his student experienced an NMAC with another aircraft as they entered the pattern. An intermittently operable COMM 2 system may have precluded their reception of traffic advisories from the Tower.

Narrative

Two main factors were that COM 2 was intermittently operating throughout flight. Traffic sensor became inoperative halfway through flight. On descent to pattern we were cleared to enter a 45 degree entry to left downwind for Runway 23R.We were two miles south of the airport and at 2;000 MSL when the incident occurred. We had just began turning downwind when we saw another aircraft directly in front coming head on and only 300 FT below. He had been given an early turn south clearance from Tower. We were aware of this; but were distracted from this due to Instructor teaching in the cockpit. Once aircraft was noticed; Instructor took control and took evasive action to avoid the other aircraft. We heard nothing from Tower in terms of traffic advisory. Upon downwind; Tower inquired if we were still on the frequency and informed that he had been attempting to call us at least three times concerning the traffic. We did not hear any of these calls; possibly due to COM 2 being inoperative. We switched to using COM 1 and continued as normal for pattern and landing.