I did some things ...
I don't see a description of what you did. A schematic would help.
Can one suggest a ready to use solution if there is one?
Here's what I use: a cheap homemade circuit to connect the audio output of the PC to the Arduino. A schematic is attached. The Thevenin equivalent of the voltage divider is a 10K resistor and a 2.5V DC supply. 10K is suitable for an analog input, as described in the datasheet. Note that there's nothing in series with the analog input. I've verified that my PC won't output a 5V peak-to-peak signal, so I don't worry about overdriving the analog input. You may prefer to use two 10K resistors, and a 5K in series, for an equivalent impedance of 10K, with the 5K providing a measure of protection for the analog input. This scheme isn't really suitable unless the input is stiff enough to drive the equivalent impedance of the voltage divider; otherwise, the signal level will drop.
I added a 100K resistor to ground from the signal input. That's because I suspect that the audio output is capacitively coupled, just like my input. Without the resistor, the DC voltage between the two capacitors is essentially undefined. Because I'm using an electrolytic capacitor, and I presume that the audio output does, too, I add the resistor to keep the DC level between them at ground, and make sure that the capacitor polarities are proper. I can't say that it's necessary; I just like having it in the circuit.
Advantages are that it's cheap, easy to build, and I can define the analog input quite precisely by generating my own audio content. The primary disadvantage is that it doesn't lend itself to real-world inputs: The input impedance is so low it would pull down most guitar inputs, and it has no protection from analog overvoltage, as you might get from an amplifier output intended for a speaker.
Criticism is welcome.