patch 8.1.2004: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
diff --git a/src/testdir/test_sound.vim b/src/testdir/test_sound.vim
index 6ffd7e6..86dc596 100644
--- a/src/testdir/test_sound.vim
+++ b/src/testdir/test_sound.vim
@@ -13,13 +13,13 @@
   if has('win32')
     throw 'Skipped: Playing event with callback is not supported on Windows'
   endif
-  let id = sound_playevent('bell', 'PlayCallback')
+  let id = 'bell'->sound_playevent('PlayCallback')
   if id == 0
     throw 'Skipped: bell event not available'
   endif
   " Stop it quickly, avoid annoying the user.
   sleep 20m
-  call sound_stop(id)
+  eval id->sound_stop()
   sleep 30m
   call assert_equal(id, g:id)
   call assert_equal(1, g:result)  " sound was aborted
@@ -35,7 +35,7 @@
   endif
 
   " play until the end
-  let id2 = sound_playfile(fname, 'PlayCallback')
+  let id2 = fname->sound_playfile('PlayCallback')
   call assert_true(id2 > 0)
   sleep 500m
   call assert_equal(id2, g:id)