Add some string to the example composition
Do not unconditionaly make synth.a from artwork

A .PHONY target will re-make each time it is mentioned as a dependency.
Remame filter.?pp to biquad.?pp

The file content is biquad related only.
Also, in the case that we may whish use it from
filters.?pp the name similarity may annoy.
Actually do zoom

Seems the actual multiplication was forgotten.
Compile with no -Wextra warnings
Move fuge to sound
Make directories as a phony target

Otherwise all gets built each time so the purpose of make is nullified.
Do not mention Makefile as dependency

When editing the makefile one will be aware of the consequences and be
ready to manually clean.  Many changed to a makefile will be to add
targets, not to fiddle with compile-flags and in that situation it is
unnecessary to re-build all.