A4 B6 Avant - intermittent sound fault

I've had a Sony S300btx for the last 4 months and there was a fault where if the volume was low the right side speakers wouldn't work and there was an occasional crackling sound, turn up the volume and the right speakers would start working. I got the car stereo installers to check it and they said it was either an audio processor fault or the audio output socket. As it was still under Sony warranty I've had it removed and sent off for inspection. I got the installers to fit a Parrot Asteroid instead a week ago but the same fault has appeared again but far less frequent than the last unit. It's a non-Bose system too.

Any ideas as to what may be the issue please?

Thanks for your help