US TV aural is frequency modulated, with +/-25 kHz deviation defined as 100%
modulation. Pre-emphasis of 75 microseconds is applied to program audio.

It is transmitted as a discrete r-f waveform about 250 kHz below the upper
edge of the TV channel. TV receivers typically demodulate it using an FM
detector tuned to the 4.5 MHz intercarrier product present in the output
signal of its video detector. Therefore if the TV visual carrier (only)
goes off the air, no audio will be heard at the TV set, either.

Any communications-type receiver capable of tuning to, and demodulating the
TV aural carrier will enable listening to it. Even some little "pocket"
AM/FM radio receivers have that ability.

RF