Do you know how a regular/manual potentiometer is wired-up?Digi-Key lists thousands of different digital pots (I'm sure they are not all in stock). 10k to 100k Ohms is a good value for line-level audio. You'll need two for stereo (or you can get a dual digital pot), but you can connect as many as you want to the same control-signals from the Arduino. You'll probably want to get a thru-hole package if you want to avoid soldering the smaller surface-mount parts.Different digital pots are controlled differently, and it's your choice of how you want to do it. The most common method is serial I2C. There is an I2C library for the Arduino, so it shoudn't be too difficult. Some are controlled with a parallel (binary data) connection. Some have up/down digital controls, which might have an advantage in that you won't get a sudden full-volume blast if there's a little "glitch" i the control data. An audio volume control has an "audio taper" (approximately logarithmic). This is because 50% on a regular (linear) pot sounds a lot closer to full-volume than half-volume (due to our logarithmic hearing). With a digital pot, this can be handled in your sketch.
If u r a hobbyist then I suggest that u buy electrical junk like old defective Radios, Stereos, TVs, VCRs ...etc.Then salvage whatever you need whenever u need it! Goodluck
Hey Jomn. I was in the same boat as you. I opted for a PT2257 to control the volume for a project I have on the go at the moment.Great way of learning, read the datasheets.Build the example circuits that most datasheets have, on a breadboard.It was a bit unfortunate that I couldn't find a lib for the PT2257. So I wrote one myself, which was another challenge for a newbie. (To be honest it's not complete, but it functions for my needs.)There are some good tutorials here....http://tronixstuff.wordpress.com/2010/10/20/tutorial-arduino-and-the-i2c-bus/Good luck with your project. It will be frustrating at times, but when you have a working example, you get a great feeling. Then you move onto the next part & start the process again.