Building it

core board:

###top side:

w/ SPI flash OR serial SRAM (depending on what it is you want to do), ADP150 regulator, codec, op amps:

note (or ignore) the pad labelled zero*: don't populate it!
the op amp labelled in is the standard inverting configuration, offset by ~ 1.65v; it serves as an attenuator. as per BOM, fb should be 27k (thus gain = 27k/100k = 0.27). the inputs are DC coupled.
the V_bias divider in that case (fb = 27k) should be formed by 47k and 30k, as shown in the picture. (ie, ignore the values on the silkscreen). ie: offset = 1.65v = (27/100 + 1) * V_bias ; where V_bias = 30/(47+30)*3.3v =~ 1.29v
the op amp labelled out is the standard non-inverting configuration; it serves an an amplifier. as per BOM, fb should be 27k, gnd 5k6. the outputs are AC-coupled.
the CV inputs (the dual op amp near the bottom of the board) are in the standard inverting configuration, too. in this case, input impedance: 100k; gain = 33k/100k(and see the note re AREF below). the inputs are offset to 3v3/2 via the lm4040-5v0 (33k/100k*5v = 1.65v).
as for RAM vs flash: as mentioned, that'll depend on what it is that you want to do: flash (16MB) will be useful for permanent storage only (e.g. drum samples); the serial SRAM has far less capacity (1Mbit) - less than 1 sec stereo @ 44k -, but can be written to much faster, suitable for audio buffers (see the delay example).

###bottom side:

note: using LM4040-3 (or 2v5) is entirely optional (ie for use as external AREF). if used, the offset will need adjustment, because the effective range reduces to 0 - 3v0. to compensate, use 110k input resistors instead: 33/110 * 5.0v = 1.5v ; ditto for lm4040-2v5. in this case, 30k and 120k would get you there: 30/120 * 5.0v = 1.25v.

you can either normal the right input to the left input (shown in picture), or keep them separate (in that case, don't jumper L -> R but the pad that says gnd):

note: the recommended (suitable? commonly available?) tact switches (TL1265) aren't quite tall enough. don't try to push them all the way through but leave some clearance (a few mm) in between the pcb and the body of the switch, as pictured below ... not too much clearance, of course, the switches shouldn't push against the panel either:

re illuminated switches, the cathode is marked with a -. as noted though, if you don't feel like LEDs, you might as well use non-illuminated buttons.