so guys, I am back on that machine! it is a model 56 486SLC2

things I did and noticed
- internal 500MB SCSI dirve working just fine! (DOS,OS/2 2.1/3.0/ WIN95 already working)
- external 2000MB SCSI drive also working fine.
- external 5,25" 360KB just working fine
- 10/100Mbit mca network card is fine in dos , has issues is win95 (perhaps the wrong driver)
- covox as scratching issues*

* https://ardent-tool.com/9556/#Speaker_Noise

I am not sure if the issue described above is the same for the LPT-Sounddevice. It's pretty clear that there is an issue on the sound-quality
and it seems that there is a connection between the video output and the noises. so what should I do? perhaps getting an MCA soundcard
will solve that, but these are very rare and expensive.