Map numpy audio band arrays to files

I've noticed that the audio band arrays can be very memory-intensive. I've seen my RAM easily eaten up just from the "analyzing audio" phase.

We could have array data on disk using `numpy.memmap`.

Locate the code that creates and consumes the audio band arrays during the “analyzing audio” phase; the issue names NumPy and `numpy.memmap` but no file or test paths. Confirm how those arrays are currently allocated, then verify that the analysis uses on-disk array data without exhausting RAM.

Written by the indexing model from the issue text.