"Sources" tend to have a low output impedance. "Inputs" ("Sinks" if you like) have a high input impedance in order to reduce circuit and cable loading. The low impedance tends to swamp electric field related noise injection with signal.

An extreme case would be connecting an open RCA patch cord to a Phono preamp input or a high impedance microphone input. It is likely that there will be hum and noise picked up from the surrounding area. On the other hand, it is not likely that an open speaker wire connected to a speaker will pickup enough energy to be an issue. (However, close to a broadcast radio transmitter I have experienced this with long speaker wire runs)