Instructor pilot reported that the communication system was not on the correct switch and they were NORDO for 15 minutes until the problem was resolved.

Narrative

On Date around XA30 Aircraft X took off of Bakersfield Airport Runway 30R. The flight took off for an annual pilot evaluation flight to include various private pilot maneuvers.Pilot flying of Aircraft X had requested and received a transponder squawk and departure frequency from ATC for local radar flight following.After Takeoff Bakersfield Tower instructed Aircraft X to contact departure. The evaluated pilot switched from COM No2 to COM No1 (transmit and receive) and contacted departure leaving 2;000 feet.Departure acknowledged with 'Radar Contact'. Aircraft X resumed flight to a practicing area approximately 10 SM SE of Bakersfield Meadows Field climbing to 5;500 feet. I; as the right seat evaluator spent most of the time between ADSB-In and outside to scan for traffic. I directed the evaluated pilot (Pilot Flying left seat) to fly certain headings to steer the flight around local area traffic. Arriving approximately 10 SE of Meadows Field at 5;500 Pilot Flying was instructed to demonstrate steep turns; slow flight; power off stall and recovery.Prior to every maneuver Pilot Flying performed clearing turns to clear the area of traffic.One of the maneuvers was a simulated engine out procedure and I instructed the pilot to close the throttle. Pilot Flying started circling gliding descent looking for a landing site and performed simulated emergency procedures. On the GPS (Garmin 400W) we got an alert - aircraft underneath us 500 feet. I was wondering why we did not have any traffic advisories for a while and checked the audio panel.I saw COM 2 Receive light lit up (COM 2 radio was ATIS in Active and Bakersfield Ground in Standby).I gave the pilot a vector away from the traffic and advised him to select COM 1 Transmit and receive. COM 1 had Bakersfield Approach in active (for radar advisories) and Bakersfield Tower in standby. I gained a visual on the airplane causing the traffic alert moving away from us 5 o'clock about 1/2 laterally and about 300 feet horizontally underneath.Pilot Flying switched to COM 1 Receive and performed a radio check. Bakersfield Approach advised us that they were trying to call us on all Bakersfield Frequencies for the past 15 minutes trying to issue us traffic advisories and alerts. Pilot apologized and remained on COM 1 transmit/receive until instructed to switch to Tower. Bakersfield Approach issued a telephone number to the pilot.Upon landing and shutdown Pilot Flying called the issued phone number and ATC told him he was off comms for about 15 minutes and Approach tried to call several times on all frequencies to issue traffic advisory and alert. ATC TRACON Controller mentioned he was watching us spiraling down and was worried.ATC TRACON controller said he was not looking for an apology but for lessons learned resultant from this NORDO situation.Lesson learned for me as Evaluator Pilot/Pilot Non-Flying: 1. Frequent checking of the audio panel setting and squelch of the sound as often as I can to prevent any future NORDO situations during Evaluation flights are a 'MUST' for me.2. Trouble shooting of the Audio Panel: 1. Transmit COM 1 or COM 2 is a turning knob: Normally: When selecting to transmit COM 1 or COM 2 on the radio the turning button has to be selected for the appropriate COM to transmit on:The 'Receive feature is an illuminated green light to the left of the panel:When selecting COM 1 to transmit normally the COM 1 receive light illuminates. FBO will check out the radio and ensure proper function of the audio panel during follow-up flight and refer to Maintenance if audio panel is not functioning as intended."