As luck would have it, I found some SD-card boards at work, and I had a bit of a epiphany. WHY am I trying to do ADC? Just so I can have a signal coming from a CD player line-out - which is another piece of equipment anyway. Playing from an outmoded storage format - an audio CD. Why am I doing this? Well, because that's the way it's done, right? Why not just have the files on a SD card, and read 'em off there. Suddenly I don't have the sampling/timing problems - because if I can read file(s) off a SD card fast enough, that solves pretty much all of the sampling problems - they will be high frequency and there will be no inaccuracy in sampling.
More to the point, why convert the original CD files to analog, then back to digital? Especially when I don't have to. So, maybe it's time to divert to adding one of these SD card boards to my arduino and trying to read files. It's not exactly old-skool NBTV but then again the use of a CD player isn't either. This could be the way to go!