patch 8.2.2959: sound_playfile() is not tested on MS-Windows

Problem:    sound_playfile() is not tested on MS-Windows.
Solution:   Make it work and enable the test. (Dominique Pellé, closes #8338)
diff --git a/src/sound.c b/src/sound.c
index 9d91f6c..702fd25 100644
--- a/src/sound.c
+++ b/src/sound.c
@@ -355,10 +355,9 @@
     if (wp == NULL)
 	return;
 
-    PlaySoundW(wp, NULL, SND_ASYNC | SND_ALIAS);
+    if (PlaySoundW(wp, NULL, SND_ASYNC | SND_ALIAS))
+	rettv->vval.v_number = ++sound_id;
     free(wp);
-
-    rettv->vval.v_number = ++sound_id;
 }
 
     void