Hardware wii hdmi video + AV audio

[Duckieh]

I've a special case where my wii, running on system 4.0, uses a Wii2HDMI adaptor like the one in the picture below to output video to my monitor.
Secondly, I connect the audio output that can be seen on this adaptor to my stereo through a jack 3,5mm stereo to RCA (AV) audio cable (second picture).
Now my problem: the stereo audio makes an incredibly constant noisy buzzing sound. Whenever i use the same cable in other hardware, there is no problem in audio.

I guess it has to do with either the adaptor or the wii audio output.
Somewhere I have seen a comment that there is an audio fix in wii IOS systems 4.2 or 4.3 when the audio is provided through hdmi, but after searching for hours i cant find it back. my wii is softmodded and i have no way to connect to the internet with it (FYI).

Is there a cheap way to both connect my wii through hdmi and still get audio through my stereo?

Greetings,
Duckieh


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[Duckieh]

PS: I tried to safely update my wii system, but startpatch is not working. Simulating an update with NUS downloader neither.
I can put much more effort in upgrading my wii system, but im not even sure if updating it will fix the audio issue!

is there an application to boost audio output from the wii, so it can dominate the buzzing sound (only amplifying sound on stereo increase buzzing, boosting sound on wii doesnt).