In this guide, I will be soldering two RCA composite jacks to a 3.5mm "aux" cable, but the process is the same for any type of audio cable you might use (e.g. XLR, 1/4", etc.).

Note: Make sure to read through each step entirely before attempting it.

Supplies

Soldering Iron & Solder

Any two audio cables

Wire Strippers

Helping Hands (optional)

Electrical or Gaff Tape

Heatshrink (optional)

Step 1: Strip the Wires

Cut your cables and strip them, making sure to remove the insulation while leaving the wires inside more or less untouched. Having only one or two strands left of a wire will result in a bad connection when you go to solder them. Make sure to cut off enough wire (1" should suffice) to leave space for the heat shrink. Once you strip off the outside, you'll want to strip off about a 1/4" off of the insulated wire inside.

Step 2: Identify Wires

Depending on what you are soldering, you will either see two or three wires inside. You should see an interior wire wrapped in more insulation, the positive voltage. There is another strand of wire surrounding the outside of that, this is the negative voltage. Match up the two audio cables so that the negative channels touch and the positive ones touch as well.

Note: if you have three wires, you are soldering a stereo audio cable. You'll need to match up the white insulated wire (left channel) with the corresponding wire on the other cable (most likely it will also be white), and do the same for the red insulated wire (right channel). Match up the negatives of both cables as well (the wire surrounding the left/right channels).

If you are soldering a stereo cable to two mono ones, you'll want to share the negative voltage between all three.

Step 3: Add Heat Shrink

In order to make sure the wires don't short, cut off a piece of low diameter heat shrink (the length of which depends on how much wire is exposed) and slot it onto each pair of matched wires. Make sure there's enough room for you to adequately solder the wires together. Alternatively, you could just use electrical/gaff tape to isolate the soldered connections (this is what I did).

Step 4: Solder the Wires

With the wires and their respective counterparts matched up, clamp them so they are touching and solder them together, heating up the wires with the iron and applying the solder so that they bond nicely. Avoid excessive movement of the joint while soldering, otherwise, you risk having a cold solder point.

Step 5: Finishing Touches

If you used heat shrink, use a heat gun/blow dryer/lighter or your soldering iron to shrink it onto the solder points. Make sure it covers all of the exposed wire.

Important: Finish it off by wrapping the whole thing in electrical/gaff tape (I personally find gaff tape easier to work with).This prevents any potential shorts between the lines.

If you hear distortion in the signal, you've either soldered the wrong channels/wires together or most likely, the negative and positive wires are touching and shorting. I had to shift around each connection and individually wrap them to protect them from this.