spkr0 is the new (as of 2021-10) audio mode config for FCDEV3B
loudspeaker+microphone voice call kit.  The other two configs are historical
and should no longer be used:

spkrmax config was my (Mother Mychaela's) concoction at the time when I tried
to use tiny cellphone-sized speakers, which were too quiet in the absence of
special mechanics and acoustics.  It needs to be noted that setting a positive
speaker "calibration" gain in the Iota ABB with the volume control set to max
and with the waveform at full scale produces clipping instead of more Vpp
output, hence this approach is not really a solution.

mcsi config was an early method of entering MCSI digital voice mode, but this
method should no longer be used, as it is broken - use AT@VSEL=1 instead.