Where is the data ultimately coming from? If it's being acquired at the PC then I would be inclined to have the playback controlled there as well, and write a small PC app that just pulled values from the data file in whatever format you chose to store them, and wrote them to the Arduino serial port; the Arduino sketch would just take data sets from the serial port as they arrive and write them to the DACs.
CSV would be a reasonable choice for the file format and also for the Arduino serial comms, unless the data is already encoded in some other format.