RS232 signal causing interference to audio line

"But the question was interference between wired signals"

What are PCB traces (tracks) if not "wired signals"?!

The mechanisms of interference between tracks on a PCB are identical to those between signals in cables!

If you place a long microphone cable close to a long mains cable you will get interference between them - in the form of a mains-frequency "hum" on the audio signal. If the mains cable is connected to loads like fluourescent lamps or thyristor dimmers, you will also get all sorts of other nasty noises!

Screening the microphone cable and using low-impedance, balanced signals helps to minimise the susceptibility of the audio to interference...

"But the question was interference between wired signals"

What are PCB traces (tracks) if not "wired signals"?!

The mechanisms of interference between tracks on a PCB are identical to those between signals in cables!

If you place a long microphone cable close to a long mains cable you will get interference between them - in the form of a mains-frequency "hum" on the audio signal. If the mains cable is connected to loads like fluourescent lamps or thyristor dimmers, you will also get all sorts of other nasty noises!

Screening the microphone cable and using low-impedance, balanced signals helps to minimise the susceptibility of the audio to interference...