Analogue

Headphone out to line in

Y-cableThis is a very cheap solution; a Y-cable will do the job.
Probably not the best way as you might have a mismatch between the two in signal level. Be careful with the volume control.
Sound quality is probably moderate.
This is excellent to start with, you can try your hand on computer based audio without getting your feet wet.

 

Sound card line out to line in.

Blue Jeans CableThe sound card does the digital to analogue conversion (DAC) and amplification to line level. A standard audio cable with RCA connectors will do the job. Sound quality depends on the quality of the sound card.

The name "RCA" derives from the Radio Corporation of America, which introduced the design by the early 1940s to allow mono phonograph players to be connected to amplifiers.

XLR

Balanced connections are the prosumer standard for decades.
The trick is to put the signal on one lead and the inverse on the other. The receiver compares the two signals and removes any differences between the two (common noise rejection).
This is the way to go if you run long cables in an electromagnetically noisy environment.

More en more consumer products feature balanced connections.
Many of these products are single ended by design so you need a balun to create balanced output. Likewise, at the receiver you need a differential amp or a transformer.
You might ask yourself if adding a balun, a differential amp or a transformer to the signal path is an improvement in sound quality.

 

Over short distances balanced I/O can be advantageous at least measurably because it helps with minor grounding problems as well as major grounding problems. For example I found out that if you hook a CD player to an integrated amp via an unbalanced cable there will often be some residual hum about 100 dB down or so. If both devices support balanced connections and you use a balanced cable, the hum will be reduced an additional 20 or more dB if the equipment is good enough so that the difference isn't masked by other noise in the system.

ArnyK - Is balanced audio worth it?

Cat 5

Long RCA cables are not recommended.
XLR (balanced) can be used for long cable runs but most home audio is RCA (unbalanced).


Analog stereo audio baluns convert to a balanced signal and back.

The balanced signal can be transmitted over Cat 5 cable.

Length can be up to 2000 feet over Cat5 depending on the product.

Cassette

You have an phone with a 3.5 mm out (headphone jack) and you want to play it over your car audio but it has a cassette deck without a line in.

Convert the audio to a magnetic field that modulates the head of the cassette recorder!