Unfortunately, SpikerBox is not calibrated in volts. That is why we removed mV units from the vertical axis in most of the graphs in Spike Recorder. AD units of SpikerBox are linearly proportional to volts.
Units can be expressed as C*V (volts multiplied by constant) where C is an unknown constant that should be determined by calibration of the device and V are volts. RMS has same units as signal.
Even though you do not know the actual value of constant C the measurements are very useful for expressing relative quantities like percentage of increase of signal activity and similar (since constant C will disappear from the equation when you divide two RMS values).
The main issue is that in order to make calibrated gears whole process must be more expensive. We made a trade-off so that we make accessible (not expensive) devices that can be in hands of more people. With those devices you can still make a lot of real experiments and learn a lot. Only experiments that require absolute measurements of voltage can not be done properly.
We guess for absolute measurements you have to buy more expensive stuff, and there are plenty of those out there on the market.