15kHz is not where the problem lays ..
The difficulty is at the low audio frequency limit, say 20Hz ..
Try to select a capacitor which Zc is below, say 1%, of the lowest impedance in the circuit: output of the audio source and input of the next stage ..
In many cases the output Line impedance is 10kΩ, so aplying the above rule at 20Hz the capacitance will be ≈800µF, and I assume that the input impedance of the next stage (in your case two resistors) >1MΩ --> to ensure this condition you will need a buffer ..
If you allow more realistic limit of 10% you can stay with 100µF coupling cap ..
Regards,
IanP