If phi is not null slide the volume up by phi each tick, else slide the volume down by plow each tick. Note: phi and plow can be zero, there will simply be no volume change.
A good test is the module `ghouls and ghosts
': if you mix up the test
for phi and plo, you won't hear the flute solo.
Scrolling mode:
1 A-4 vol +5
sample number / note / slide increment