Which radio altimeter is the Boeing 737NG call outs based on?
The FCOM (volume 2, 15.20.17) states:


  The GPWS provides height callouts based on altitude set by the
  Captain's Minimums selector.


According to the investigation of Turkish Airlines 1951, the captain's and FO's radio altimeter are feed independently, and the autopilot is wired to react to whatever is happening on the captain's side. This includes the aural warnings.

Curiously, I cannot locate anywhere in the FCOM, FCTM nor QRH what happens when the RAs disagree. The only related information I find is, to use the autopilot for landing you'd need both RAs to be functional. If one of them fails while the autopilot is in landing mode (i.e. glideslope and/or localizer mode), the autopilot disconnects.