Welcome to Beoworld! :)

If you have a "noisy" soundcard in your PC or if it makes plopping sound when switching on/off you shoud use some kind of DAC / preamp / USB-Audiointerface. They give you more comfort regarding volume control, too.

If you have analog outputs on your PC you could get an old tc electronic Desktop Konnekt 6. They are extremely cheap nowadays, provide very good quality and control. If you have a firewire connector on your computer you can use it as an audio interface, too:
http://www.tcelectronic.com/desktop-konnekt-6/

A similiar but pure digital solution exists from tc electronic and is called BMC-2. It's a DAC/preamp/Monitorcontroller and accepts various digital input formats:
http://www.tcelectronic.com/bmc-2/

Otherwise you can use every USB-Audiointerface that fits your needs:
https://www.thomann.de/gb/usb_audio_interfaces.html?viewMode=block

You just should have a look if it has a knob for output volume control. Than you don't have to use software to adjust the volume of your speakers. A simple and powerful interface is e.g. Novation audio hub.

or just use monitor controllers between your computer and Beolabs:
https://www.thomann.de/gb/search_dir.html?sw=monitor+controller&viewMode=block

Steve from soundsheavenly can sell you every cable you need.

I used beolab 18er speakers.  As there is no volume control in the speakers, it is worth investing in a USB dac with volume control. I used a IFI IDSD Nano and was happy with the sound. Going straight from PC/Mac to speakers means relying on software volume control and can cause moments where the speakers see full volume which is not good for relationship with neighbors nor speaker units