Sound Card 
A sound card has holes (ports) for connecting a microphone, speakers or headphones, and an outside sound source with a single prong plug. There is also a serial port for connecting devices like musical keyboards and synthesizers.
Recent devices are color-coded to help you match the connector to the correct port. Audio line IN is blue, microphone is pink, Audio line OUT (speakers) is lime green.
Some sound cards do not have the plug-in holes colored
or even marked as to which is which. If you can't find the documentation that came with the sound card, you'll have to experiment to see which one your speakers go in.
Once you figure it out, mark the holes with fingernail polish or something so you won't have the experience of working for hours to "fix" your sound when the only problem is that the speakers are plugged into the wrong hole. (Personal experience is talking here!)
Some sound cards have a volume control wheel but others rely on software volume controls. Some kinds of speakers have volume control knobs or slides.
If you want a manual control and your speakers don't have one, you can buy a device that you can reach easier than the back of the computer and faster than on-screen volume controls. It doesn't seem to have a particular name. The speakers plug into this simple device which is basically a knob to turn. Then the device plugs into the sound card's hole for speakers. Stick the knob device onto the side of the monitor or your desk and you're all set.