How to calculate "effective source impedance" in this audio mixing circuit?
Remember that the effective impedance of the signal sources is approximately zero.  Modern audio devices will often have a 47R or 100R resistor connected in series with the output (internal to the device) which (usually) makes the output impedance of that device equal to that resistor value.

Now look at the wiper of one pot.  The bottom end of the pot is grounded.  The top of the pot has an AC impedance of approximately zero.  The highest resistance point is therefore when the wiper of the pot is midway between the top and bottom of the resistor element.  That means that the approximate resistance from either the top or the bottom of the post is half of the pot value.

If you draw the pot as two resistors of about equal value, and if you model the ends of the resistors as having a source impedance of about zero ohms, you will see that the wiper connection has an impedance value of (about) the top resistor portion of the resistor track in parallel with the bottom resistor portion.  The net impedance as seen at the wiper is therefore about 1/4 of the total pot value.

This impedance is maximum when the wiper is in (about) the middle position and becomes smaller towards zero ohms as you rotate the pot towards ground or towards the hi-side (the input signal).

Note that the above is accurate only if the input to the pot is actually connected to an audio source.  If the input is disconnected, the impedance at the wiper is maximum when the pot is turned all the way towards the input signal.  That impedance is, obviously, the pot value.