The easiest way to work with one channel of a stereo pair is to split the pair into two separate tracks. See: http://manual.audacityteam.org/o/man/splitting_and_joining_stereo_tracks.html
To work with stereo tracks in Nyquist, use the AREF command http://www.cs.cmu.edu/~rbd/doc/nyquist/part19.html#index1385
See here for more information: http://wiki.audacityteam.org/wiki/Nyquist_Plug-ins_Reference#Stereo_Tracks
By the way, how are you getting on with: https://forum.audacityteam.org/t/beats-per-minute/33202/12