It seems that ddr can detect stereo and mono files, so maybe we can do away with the stereo argument.
sequence(list(piano$A2), list(beat.dynamics[1:100]), stereo = F)
| | 0%Error in FUN(X[[1L]], ...) :
One 'Wave' object is mono, the other one stereo
Calls: sequence ... reduce_waves -> Reduce -> f -> bind -> lapply -> FUN