Hissing sound then muting in only the Audio? Under those circumstances the only way you can get that is your Audio settings are wrong. If you have a GOOD picture, and hissing Audio, then you may be on Digital Audio channel 2 to 8, or maybe you activated AD audio -- Descriptive Audio on a TV show that has no Descriptive Audio.
You really can't have BAD Audio with GOOD Picture and hear hissing, because hissing is NOISE, or lack of audio signal completely.
That describes being on the wrong audio channel.
But two boxes....that's even odder. And Scart too....
Boxes send out RAW Audio to the TV so it has no control over what is delivered...but when two boxes have the same issue, I start wondering if the TV is on an RF TV channel instead of a Video Cable based connector like Scart or HDMI.
Using channel 3 or 4 or whatever channel is used with freeview boxes to connect them to TVs using RF cable would affect 2 boxes the same way.
But Scart....one box should have been bad, the other good. The same with 2 Scart cables. If you changed Scart inputs to another...that would have created a difference in audio too.
It's pointing to the TV set, but acts like it either locked up or got a power surge on it and damaged the audio.
So....is Scart and RF (antenna overload) inputs the only choices you have?
Did that 2nd box work fine on your 2nd TV set??
I just find it odd that only the SOUND is bad.