fix melody for voices 3 and 4

fix melody for voices 3 and 4

15 months later, I took another look at why the music was off for
channels 3 and 4 and pretty fast noticed that it affects only the voices
using the mod3 subroutine. Executing mod3 with simavr[1] showed that the
compare-branch step is broken. Switching BRPL for the correct BRCS fixed
the problem.

[1]: in the hackiest way imaginable:
  gdb> layout regs
  gdb> set $r24 = 0x15  # test value for Mh
  gdb> set $r23 = 0x16  # test value for Ml
  gdb> set $pc = 0x1a   # start of mod3 routine
  gdb> ni  # repeatedly