That was the case with mine, until I decided to do something about it. Instead of ordering a new one, I wanted to modify the cable myself and save a little time and money.
Reading some forum info, and getting in touch with Robert at retrogamingcables (where I initially bought this from a little while ago) it was concluded that the issue was to do with GROUND sharing.
The SNES has 2 ground connections, but in my cable only one of them was connected to the SCART end which was being used for everything else as well.
Robert informed me that my SCART cable had an unconnected yellow wire (composite video) which is redundant in this cable, that I can used as a new separate ground wire for the audio.
Simple enough...onto the fix then!
Getting into the MultiAV plug is a bit tricky, but with a little patience, you can open up the housing fairly cleanly.
I used a box-cutter, and a sharp x-acto knife.
First, score the side of the multiAV plug along the glued crease on both sides with your x-acto knife:
This is a time consuming step, but if you want the cut to be clean, then I suggest scraping away the plastic multiple times to get a deep enough crevice. You want this on both sides (I use this method when cutting plexi/lexan for arcade bezels -- score and crack)
Scratched away enough material on one side so you can cut right through into the housing. Next, insert the Box-cutter blade and carefully and slightly twist it, to free up the sides from one another, this should crack the housing along the scored lines:
If you scored cleanly enough, it should be a straight enough break once you've opened it. Remove the housing completely so you can get at both side's pins:
You will need to de-solder PIN 9 from the MultiAV and solder into the spare ground PIN 5.
Then on the scart plug end take the yellow wire which is now connected to the spare ground, and solder to PIN 4 on the SCART end. I put some heat-shrink tubing to insulate it.
After that, put the Multi AV housing back on, glue it or tape it together, whatever you prefer. The results will be quite a difference in audio noise. Though minimized, I noticed there is still a little bit of hissing there, but it is definitely a lot better. I could probably reduce this even more if I used shielded wire for the audio connections, but that would require me to radically change the cable, which I didn't really want to do at this point. But for now this works fine.
I hope this helps!
