Check whether the batteries or Accupack (BA 2015) are working. If not, replace the batteries or recharge the Accupack. Alternatively, make sure that the cable and power cords are properly connected to the main unit.
Make sure the transmitter and receiver are on the same channel and frequency if you cannot get an RF signal. Alternatively, adjust the squelch threshhold if it appears the transmitter is out of range. Turn the knob to try each of the three squelch settings - low, middle or high, until you get the best sound.
Make sure that the mute button is not on if the RF signal is available, but no audio signal is getting through. Alternatively, check whether the receiver's squelch threshold is adjusted too high or if the transmitter is not transmitting the pilot tone.
Check that the transmitter sensitivity or the receiver's AF output level have not been adjusted too low if the audio signal has too much background noise. On the other hand, if the audio signal is distorted, check that the transmitter sensitivity or the receiver's AF output level have not been adjusted too high.