I've been wanting to connect 2 devices to my amplifier (Computer and nintendo switch) so i bought an analog Audio switch with 3 channels so i could plug them both into the same amp and switch between them as needed. The sound works on my computer and Nintendo flawlessy until both of the devices are plugged into my monitor sinultaneously (Computer through hdmi and nintendo thorugh and active hdni to displayport adaptor)
When Both devices are plugged in with video and sound my speakers start hissing loudly, rendering them useless.
My setup:
Laptop --> usb c cable --> Dell wd15 docking station --> a/usb b cable --> External Sound card usb --> Phono cable --> Analog Audio switch --> Phono cable --> Amp
Nintendo --> Aux plug/phono plug --> analog audio switch --> Amp

Does anyone have the slightes idea what might cause this issue?

 

Your setup is causing "Ground-Loop" problems / Oscillations.
The inputs, in this particular case, must be "Galvanically-Isolated",
witch may not be worth the trouble and expense that it will take to fix the situation.
.
.
.

 

I see at least four devices that probably have switching mode power supplies, and most switching mode power supplies generate some noise. In addition, your "3 channel analog switch" will, if it contains any electronics, may also be a noise source.
Aside from that, the Dell computer and docking station may have noise issues all their own.
Locating noise sources in an arrangement like that can be a real challenge.

 

The solution may be to discard the switch or make your own so that you can be sure all the conductors associated with each signal connect and disconnect cleanly.

But before that make sure nothing has the gain cranked all the way up.

 

A simple mechanical switch could enable the selection of either source of the sound, but given the unknown source of the hiss, I suggest switching both the signal and the common from each source, in case the noise is a ground mode signal. That means that the switch will need to have at least 2 poles.
Of course, that is guessing that each source plays as desired when it is directly connected.