Possible Homemade RCA to Aux

If the Arduino is anything like a PIC µC then you have no hope of sampling at 44KHz. Most simple µC have quite a slow sampling rate (like 100's of samples per second).

If you want faster then you'd be looking at using something like a dsPIC which has an actual audio grade ADC in it, or use an audio ADC externally that can send I²S data to a µC that is fast enough to respond to it.

I have done some similar work recently while designing a digitally controlled amp.

I had the output of the first stage of the amp going into an analog input on the controlling PIC to then control a bargraph of LEDs for a simple VU meter.

For an output from a PC soundcard you're probably looking at around 1 to 2 volts voltage swing. For my system I wasn't too fussed about frequency and such - just pure peak amplitude - so I passed the signal through a small shottky diode first to trim off the negative voltages. This simplified my design a whole lot.

I am also designing a small frequency analyzer at the moment, and am looking at having selectable op-amp based band-pass filters based around this design: http://www.wa4dsy.net/robot/bandpass-filter-calc which so far has given quite good results. I am varying some of the resistor values by a combination of digital pots and analog multiplexers.

I would certainly recommend at least protecting your analog input(s) with op-amps to limit the maximum voltage they get - just in case. You don't want a voltage spike blowing up your Arduino now do you? Easier to replace a blown op-amp.

And as for a signal for testing? There are many free signal generators for the PC available for download if you do a little google for them. They will let you select waveform, frequency, amplitude, phase, etc. Even allow summing of waveforms to give new waveforms if you're lucky.

You can even use your PC soundcard as a rudimentary scope as well with the right software and a small home-made probe. There is software and designs around for this too on the net.

Oh, and remember to isolate different stages / voltage levels with capacitors in the audio signal. As a rule of thumb, if I am changing PSU voltage levels, I always introduce a capacitor to isolate the stages. So, I had one on the input signal, one on the stage 1 -> stage 2 (+/-5V to +/-12V power supply), one on the stage 1 -> analog input, and one again on the output. It pays to take no chances with stray DC offsets wandering into the wrong part of the circuit.

well, in general, you should have right, left, and mic, with a ground for each on the iphone end, and all 3 will have a common ground. for the headphones, right and left, with a ground for each, and ground is shared.

you will want to remove the protective coating with a microtorch or other heat source, solder and heatshrink, as opposed to twist together. You will also want to look up the pinout for the iphone side of it, and use a continuity tester to identify what each cable usually is.

red is USUALLY right, blue or green is is usually left - the white on the sennheiser is unusual (i double check this either by following the cable to the end, or using a temporary connection with crocodile clips between a prepared, tinned end of the headphone cable and jack).

Red to red, white to green, and 'copper' to 'copper' should do the trick. all 3 have the pesky coating, so you'll need to remove it.

this is the exact process/tools that have worked for me in the past - but instead of the jack, tin both ends, press to check, then solder.