Tap signal from TDA2822L, convert to line and instrument level
Fluff has covered it pretty well. I would make the following suggestions:


Leave the speaker connected in the instrument. It will be useful for checking the "instrument" but, more importantly, may help retain some of the distortion due to the non-linearity of the speaker.
The speaker has neither terminal connected to ground but you can take your tap-off between one output and ground - i.e., either pin 1 or 3 to 4.
Since the output pins 1 and 3 will have half-supply voltage on them in the quiescent state you will need the DC blocking capacitor as you've stated. Make it big enough to avoid too much bass roll-off with the pot resistance. Use http://sim.okawa-denshi.jp/en/CRtool.php to assist.
Remember for best signal to noise ratio (or, in this case, desired noise to unwanted noise ratio) you will want the volume up high on the instrument and your output divider will then be used to attenuate the signal and the noise.