Interfacing an RCA audio output to a microcontroller ADC
Here is something to try:



Audio signals meant to go between equipment but not directly from microphones or meant to drive a speaker are usually at line level, which is usually around 1 V RMS.  That is about the right amplitude for driving a microcontroller A/D input, but the DC level needs to be set close to half of the A/D range.  This is what R2 and R3 do in this circuit.

C1 decouples the DC level of the input signal from that at the A/D input.  R1 provides a little optional attenuation in case the audio level is too high and ends up clipping against the A/D limit too often.

Note that R2 and R3 together provide 50 k&Omega; of impedance as seen by the A/D input.  The high pass filter formed by C1 and this impedance has a rolloff of 16 Hz, which is low enough to pass "HiFi" audio acceptably.