What for are two resistors and one capacitor when attaching an audio CD drive output to a TDA7318, but not for Line In?
The tuner is built into the unit, utilizing a Philips OM5610 module.
The datasheet shows (page 11, pin 9 and 11) that this module has the audio coupling/DC blocking capacitors built in so there's no need to add additional ones.
The other audio inputs are connectors to external devices. One cannot make assumptions about external devices so it's best to add the capacitors to block any possible DC on those inputs.
The 2 resistors on the CD player input form a resistor divider that attenuates the incoming signal amplitude by approximately -1.7dB. Why is anyone's guess. Perhaps the author's CD player is a bit too loud.