Succinct answer: I hate ISD too. There is cause enough for this merely
in their (unstated) datasheet errata, let alone the actual chip specs.
I've been asked recently to do some similar projects (vending machine
applications, mostly). I've chosen to use an SD card (in MMC/SPI mode)
to store raw PCM samples. I use an Atmel ATmega8 with an R-2R on one of
the ports to do the DACing. Using a parallel 16-bit DAC seems to be an
intractable problem on an 8-bit micro because of the impossibility of
updating both bytes at the same time.
I've found the Mega8 fast enough that you can bit-bang your 16 bit
samples out fast enough into a PCM56P (must try out some of those new
tiny serial DACs that AD and Maxim are putting out these days). The ISD
data sheets are a masterpiece of misdirection but at least they
basically work, unlike the Invox clones that came out a few years ago
(then vanished). The CS4231/AD1848 codec chips do 16 bit mono, stereo
etc and have a byte wide interface (have used these in combination with
30 pin SIMMS for digital delays etc).
M
M