diff -Naur wmix-3.4.orig/include/mixer.h wmix-3.4/include/mixer.h
--- wmix-3.4.orig/include/mixer.h	2018-04-30 14:17:50.000000000 +0200
+++ wmix-3.4/include/mixer.h	2020-01-29 19:07:14.932404252 +0100
@@ -55,28 +55,28 @@
  * - Muting must occur independently of the volume level.
  */
 
-void (*mixer_init)(const char *mixer_device,
+extern void (*mixer_init)(const char *mixer_device,
                    bool verbose,
                    const char *exclude[]);
-bool (*mixer_is_changed)(void);
-int (*mixer_get_channel_count)(void);
-int (*mixer_get_channel)(void);
-const char *(*mixer_get_channel_name)(void);
-const char *(*mixer_get_short_name)(void);
-void (*mixer_set_channel)(int channel);
-void (*mixer_set_channel_rel)(int delta_channel);
-float (*mixer_get_volume)(void);
-void (*mixer_set_volume)(float volume);
-void (*mixer_set_volume_rel)(float delta_volume);
-float (*mixer_get_balance)(void);
-void (*mixer_set_balance)(float balance);
-void (*mixer_set_balance_rel)(float delta_balance);
-void (*mixer_toggle_mute)(void);
-void (*mixer_toggle_rec)(void);
-bool (*mixer_is_muted)(void);
-bool (*mixer_is_stereo)(void);
-bool (*mixer_is_rec)(void);
-bool (*mixer_can_rec)(void);
-bool (*is_exclude)(const char *short_name,
+extern bool (*mixer_is_changed)(void);
+extern int (*mixer_get_channel_count)(void);
+extern int (*mixer_get_channel)(void);
+extern const char *(*mixer_get_channel_name)(void);
+extern const char *(*mixer_get_short_name)(void);
+extern void (*mixer_set_channel)(int channel);
+extern void (*mixer_set_channel_rel)(int delta_channel);
+extern float (*mixer_get_volume)(void);
+extern void (*mixer_set_volume)(float volume);
+extern void (*mixer_set_volume_rel)(float delta_volume);
+extern float (*mixer_get_balance)(void);
+extern void (*mixer_set_balance)(float balance);
+extern void (*mixer_set_balance_rel)(float delta_balance);
+extern void (*mixer_toggle_mute)(void);
+extern void (*mixer_toggle_rec)(void);
+extern bool (*mixer_is_muted)(void);
+extern bool (*mixer_is_stereo)(void);
+extern bool (*mixer_is_rec)(void);
+extern bool (*mixer_can_rec)(void);
+extern bool (*is_exclude)(const char *short_name,
                    const char *exclude[]);
-void (*mixer_tick)(void);
+extern void (*mixer_tick)(void);
diff -Naur wmix-3.4.orig/wmix.c wmix-3.4/wmix.c
--- wmix-3.4.orig/wmix.c	2019-07-12 18:44:38.000000000 +0200
+++ wmix-3.4/wmix.c	2020-01-29 19:06:52.685402994 +0100
@@ -53,6 +53,31 @@
 static int mouse_drag_home_y;
 static int idle_loop;
 
+void (*mixer_init)(const char *mixer_device,
+                   bool verbose,
+                   const char *exclude[]);
+bool (*mixer_is_changed)(void);
+int (*mixer_get_channel_count)(void);
+int (*mixer_get_channel)(void);
+const char *(*mixer_get_channel_name)(void);
+const char *(*mixer_get_short_name)(void);
+void (*mixer_set_channel)(int channel);
+void (*mixer_set_channel_rel)(int delta_channel);
+float (*mixer_get_volume)(void);
+void (*mixer_set_volume)(float volume);
+void (*mixer_set_volume_rel)(float delta_volume);
+float (*mixer_get_balance)(void);
+void (*mixer_set_balance)(float balance);
+void (*mixer_set_balance_rel)(float delta_balance);
+void (*mixer_toggle_mute)(void);
+void (*mixer_toggle_rec)(void);
+bool (*mixer_is_muted)(void);
+bool (*mixer_is_stereo)(void);
+bool (*mixer_is_rec)(void);
+bool (*mixer_can_rec)(void);
+bool (*is_exclude)(const char *short_name,
+                   const char *exclude[]);
+void (*mixer_tick)(void);
 /* local stuff */
 static void signal_catch(int sig);
 static void button_press_event(XButtonEvent *event);