thank you for the information. so would i just connect the leads from a female 3.5mm plug to the arduino?
No... You need a protection circuit! Audio signals are AC. They swing positive and negative. The Arduino can be damaged with negative voltages. A headphone signal could also go over 5V peak at high volumes and that can damage the Arduino too.
It's actually unlikely that you get more than 5V unless you connect a power amp, but eliminating that part of the protection circuit will only eliminate one diode. And, when you compare the cost of a diode to the cost of the Arduino... It's up to you to figure-out if it's worth it. 
P.S.
By using this particular protection circuit, we are throwing-away the negative half of the audio waveform. But since the positive and negative halves are roughly equal, we don't need the negative-half to "read" the volume.