So I have this nice set of Beolab 14.4 - but as my other B&O equipment already has speakers, I thought about connecting it to my PC, just in a 14.2 setup (not going to use the last two sat speakers)

So far so good, I have also bought some male mini jacks, so it is possible to connect that multi-cable which comes with the Beolab14, with 3rd part equipment.

That is also working and I get sound, the problem is, I get a humming sound during playback, and when I turn off the PC, the low humming noise becomes very loud. So I have disconnected the speaker again, and back with my good old pc speaker setup with 2.1

So the question next here is, as I am obviously doing something wrong here, what is the correct way of connecting Beolab 14 to a PC - can it be done in a proper way?

Some other information: In the normal PC speaker setup in a 2.1 there's no humming or other noises at all. the noise is only when connecting Beolab 14.