Fix +sound on macOS builds

[ychin]

v9.0.700 (#11283) accidentally turned off the +sound feature on macOS because it removed the FEAT_BIG ifdef but didn't scrub the existing usages. Fix sound feature to depend on HUGE instead of BIG.

[codecov]

Codecov Report

Merging #11497 (12c032c) into master (c57b5bc) will decrease coverage by 8.74%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11497      +/-   ##
==========================================
- Coverage   81.82%   73.07%   -8.75%     
==========================================
  Files         162      158       -4     
  Lines      189752   185528    -4224     
  Branches    43137    42538     -599     
==========================================
- Hits       155261   135574   -19687     
- Misses      21925    37919   +15994     
+ Partials    12566    12035     -531     

Flag	Coverage Δ
huge-clang-none	?
huge-gcc-none	54.67% <ø> (+<0.01%)	⬆️
huge-gcc-testgui	53.12% <ø> (+0.01%)	⬆️
huge-gcc-unittests	?
linux	55.08% <ø> (-27.44%)	⬇️
mingw-x64-HUGE	76.43% <ø> (-0.01%)	⬇️
mingw-x86-HUGE	77.26% <ø> (+<0.01%)	⬆️
windows	78.06% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/os_unix.c	0.00% <0.00%> (-66.88%)	⬇️
src/hardcopy.c	12.19% <0.00%> (-64.87%)	⬇️
src/xdiff/xpatience.c	17.24% <0.00%> (-64.75%)	⬇️
src/if_tcl.c	20.51% <0.00%> (-61.13%)	⬇️
src/if_cscope.c	12.33% <0.00%> (-56.30%)	⬇️
src/gui_gtk_x11.c	0.52% <0.00%> (-51.11%)	⬇️
src/if_ruby.c	40.20% <0.00%> (-44.00%)	⬇️
src/term.c	33.16% <0.00%> (-40.71%)	⬇️
src/crypt.c	37.16% <0.00%> (-37.84%)	⬇️
src/xdiff/xhistogram.c	40.36% <0.00%> (-33.36%)	⬇️
... and 136 more

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