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.
If this is indeed an issue of analog/digital separation there are a number of electrical strategies to overcoming it. The easiest of these is to have a separate ground plane for your analog circuitry from your digital cicuitry and join those ground planes at the common return (i.e. the power input to your board). This is called a star-ground topology. I'm not sure what you're going for with the diodes you mention in your question, but you might also consider filtering the VCC to your analog circuit through a ferrite bead and capacitor, and make sure you have adequate decoupling capacitors near all the VCC pins of your amplifiers and other analog circuits.
Best Answer
Manual here such as it is.
Disconnect microphone wires AFTER noting which wire goes where.
A photo that is detailed enough and sharp enough to allow you to tell is a good idea.
Putting a copy of the photo on disk somewhere with a file name that tells you where it is "is wise" Emailing a copy to yourself using a persistent online email account (eg gmail and many more) with key words in the message and subject line is overkill and latterly often welcome.
Connect 2 wires to where mic went with a 100 uF electrolytic cap in one wire.
Try connecting phone or line audio source to the above input - start with volume turned down, and turn up progressively while listening to result.
If level is far too high add two resistors to form an attenuator. Value shown below are a start - abut 11:1 attenuation.
Attentuation = R2/(R1+r2) if capacitor impedance is small compared to resistor values.
Making values too low (eg 10 Ohm, 1 Ohm) loads input excessively and capacotor impedance may be too high. .
Making values too high eg 1 megOhm/100k causes impedance matching problems at recorder.
R1 and R2 could be a single eg 10k potentiometer with output at wiper.
simulate this circuit – Schematic created using CircuitLab