To add to what has beem posted, pro audio uses different reference levels- if you look at the specs, you'll often see +4dBU and consumer levels at -10dB. The link explains this.

[Link: en.wikipedia.org]

Here's an explanatin of 'balanced audio'-

[Link: en.wikipedia.org]

Best practice when sending unbalanced audio to a balanced input- use something that allows the impedance of the output to match the input where it's connected. Some mics are high impedance, some are low impedance- Low impedance mics use the XLR jacks at the input of a mixer, high impedance have a 1/4" plug. Microphone manufacturers often have a small transformer that converts between these and it connects at the mic, then the appropriate cable is connected normally.

For unbalanced audio sources, like SammPX's example, that Art piece is great. If you have multiple sources that need to be connected, it may be simpler and cheaper to use a simple matching transformer for each and mount them on some kind of panel. Jensen Transformers and EDcor make these, as well as a lot of other companies. Go to the PA/recording area of a music store and you should be able to find what you need.