I personally have a lot of headphones around the house that I don't use in most cases, and the main reason for leaving them lying around is that they don't have their own volume control. From Lifehacker I find a simple hack to make us a universal volume control for our headphones.
As you can see, the trick is to replace the speakers themselves with a medium jack connector in headphones that already have a volume control. Here are the diagrams for the two ways to do it, replacing only the speakers, or placing both the volume control and the jack connector:
If you need the full manual, here you have it in English.
English 
Spanish
Hello, I would like to know if the output of the amplifier goes to the potentiometer cursor and the output of the potentiometer is made from the end, that is to say on the total value of the potentiometer.
hello i would like to do the same but for a music system of about 20 or 30 whats will it hold? or will i have to do it with larger capacity components? thanks.
I don't understand how to read that =/ why are the wires joined and separated in the diagram?
Thank you in advance
Be very careful, the resistive value of the volume control may burn out the output components of the amplifier of the device.
hola como estas, serviria para ponerlo en dos parlantes de equipo de musica , asi no tener que tener encendido el equipo de musica para que funcionen, es que cuando arme un cable para conectar directamente los parlantes del equipo de musica a la pc , se escuchaba muy bajo el volumen, chau gracias