diff --git a/src/bgi.c b/src/bgi.c
index 12e8d52..f9a9f9d 100644
--- a/src/bgi.c
+++ b/src/bgi.c
@@ -48,14 +48,14 @@ static char *bgi_gets(char *buf, int n, FILE *fp) {
 	return buf;
 }
 
-int bgi_read(const char *path) {
+bool bgi_read(const char *path) {
 	if (!path)
-		return NG;
+		return false;
 
 	FILE *fp = fopen(path, "rb");
 	if (!fp) {
 		WARNING("Could not open %s", path);
-		return NG;
+		return false;
 	}
 
 	char buf[100];
@@ -72,7 +72,7 @@ int bgi_read(const char *path) {
 		}
 		bgi_nfile++;
 	}
-	return OK;
+	return true;
 }
 
 bgi_t *bgi_find(int no) {
diff --git a/src/bgi.h b/src/bgi.h
index 7589c0d..0ede362 100644
--- a/src/bgi.h
+++ b/src/bgi.h
@@ -23,6 +23,8 @@
 #ifndef _BGI_H__
 #define _BGI_H__
 
+#include <stdbool.h>
+
 typedef struct {
 	int no;      // シナリオ上での曲番号
 	int loopno;  // ループ回数 (０の場合は無限)
@@ -30,7 +32,7 @@ typedef struct {
 	int len;     // 曲長さ (サンプル数)
 } bgi_t;
 
-extern int bgi_read(const char *path);
+extern bool bgi_read(const char *path);
 extern bgi_t *bgi_find(int no);
 
 #endif /* _BGI_H__ */
diff --git a/src/bgm.emscripten.c b/src/bgm.emscripten.c
index d0d3647..b3722eb 100644
--- a/src/bgm.emscripten.c
+++ b/src/bgm.emscripten.c
@@ -26,38 +26,34 @@
 #include "bgi.h"
 #include "sdl_core.h"
 
-int musbgm_init(DRIFILETYPE type, int base_no) {
+bool musbgm_init(DRIFILETYPE type, int base_no) {
 	if (type == DRIFILE_BGM)
 		return bgi_read(nact->files.bgi);
 	else
 		EM_ASM({ xsystem35.cdPlayer.setBGMLoader($0, $1); }, type, base_no);
-	return OK;
+	return true;
 }
 
-int musbgm_exit(void) {
+void musbgm_exit(void) {
 	musbgm_stopall(0);
-	return OK;
 }
 
-int musbgm_reset(void) {
+void musbgm_reset(void) {
 	musbgm_stopall(0);
-	return OK;
 }
 
-EM_JS(int, musbgm_play, (int no, int time, int vol, int loop_count), {
+EM_JS(bool, musbgm_play, (int no, int time, int vol, int loop_count), {
 	xsystem35.cdPlayer.fade(time * 10, vol / 100);
 	xsystem35.cdPlayer.play(no, loop_count == 0);
-	return xsystem35.Status.OK;
+	return 1;
 });
 
-EM_JS(int, musbgm_stop, (int no, int time), {
+EM_JS(void, musbgm_stop, (int no, int time), {
 	xsystem35.cdPlayer.stop(time * 10);
-	return xsystem35.Status.OK;
 });
 
-EM_JS(int, musbgm_fade, (int no, int time, int vol), {
+EM_JS(void, musbgm_fade, (int no, int time, int vol), {
 	xsystem35.cdPlayer.fade(time * 10, vol / 100);
-	return xsystem35.Status.OK;
 });
 
 int musbgm_getpos(int no) {
@@ -75,20 +71,19 @@ int musbgm_getlen(int no) {
 	return bgi->len / 441;
 }
 
-int musbgm_isplaying(int no) {
+bool musbgm_isplaying(int no) {
 	int t = EM_ASM_INT_V( return xsystem35.cdPlayer.getPosition(); );
 	return ((t & 0xff) == no);
 }
 
-int musbgm_stopall(int time) {
-	return musbgm_stop(0, time);
+void musbgm_stopall(int time) {
+	musbgm_stop(0, time);
 }
 
-int musbgm_wait(int no, int timeout) {
+void musbgm_wait(int no, int timeout) {
 	for (int i = 0; i * 16 < timeout * 10; i++) {
 		if (!musbgm_isplaying(no))
 			break;
 		sdl_wait_vsync();
 	}
-	return OK;
 }
diff --git a/src/bgm.h b/src/bgm.h
index 3c85df9..1c5d57a 100644
--- a/src/bgm.h
+++ b/src/bgm.h
@@ -19,18 +19,19 @@
 #ifndef __BGM_H__
 #define __BGM_H__
 
+#include <stdbool.h>
 #include "ald_manager.h"
 
-int musbgm_init(DRIFILETYPE type, int base_no);
-int musbgm_exit(void);
-int musbgm_reset(void);
-int musbgm_play(int no, int time, int vol, int loop_count);
-int musbgm_stop(int no, int time);
-int musbgm_fade(int no, int time, int vol);
+bool musbgm_init(DRIFILETYPE type, int base_no);
+void musbgm_exit(void);
+void musbgm_reset(void);
+bool musbgm_play(int no, int time, int vol, int loop_count);
+void musbgm_stop(int no, int time);
+void musbgm_fade(int no, int time, int vol);
 int musbgm_getpos(int no);
 int musbgm_getlen(int no);
-int musbgm_isplaying(int no);
-int musbgm_stopall(int time);
-int musbgm_wait(int no, int timeout);
+bool musbgm_isplaying(int no);
+void musbgm_stopall(int time);
+void musbgm_wait(int no, int timeout);
 
 #endif /* __BGM_H__ */
diff --git a/src/bgm.sdlmixer.c b/src/bgm.sdlmixer.c
index a62b7b7..bdbdf23 100644
--- a/src/bgm.sdlmixer.c
+++ b/src/bgm.sdlmixer.c
@@ -78,30 +78,28 @@ static Mix_Music *bgm_load(int no) {
 	return mix_music;
 }
 
-int musbgm_init(DRIFILETYPE type, int base) {
+bool musbgm_init(DRIFILETYPE type, int base) {
 	dri_type = type;
 	base_no = base;
 	if (type == DRIFILE_BGM)
 		return bgi_read(nact->files.bgi);
-	return OK;
+	return true;
 }
 
-int musbgm_exit(void) {
+void musbgm_exit(void) {
 	free_music();
-	return OK;
 }
 
-int musbgm_reset(void) {
+void musbgm_reset(void) {
 	free_music();
-	return OK;
 }
 
-int musbgm_play(int no, int time, int vol, int loop_count) {
+bool musbgm_play(int no, int time, int vol, int loop_count) {
 	if (current_no)
 		musbgm_stop(current_no, 0);
 
 	if (!bgm_load(no))
-		return NG;
+		return false;
 
 	// We don't use the loop information in the BGI file, but SDL_mixer
 	// understands loop info in the WAVE's "smpl" chunk.
@@ -109,26 +107,24 @@ int musbgm_play(int no, int time, int vol, int loop_count) {
 	Mix_VolumeMusic(vol * MIX_MAX_VOLUME / 100);
 	if (Mix_FadeInMusic(mix_music, loop_count == 0 ? -1 : loop_count, time * 10) != 0) {
 		free_music();
-		return NG;
+		return false;
 	}
 	start_time = SDL_GetTicks();
 
-	return OK;
+	return true;
 }
 
-int musbgm_stop(int no, int time) {
+void musbgm_stop(int no, int time) {
 	if (no == current_no)
 		Mix_FadeOutMusic(time * 10);
-	return OK;
 }
 
-int musbgm_fade(int no, int time, int vol) {
+void musbgm_fade(int no, int time, int vol) {
 	if (no != current_no)
-		return NG;
+		return;
 
 	// SDL_mixer doesn't provide arbitrary fading, so just set the volume immediately.
 	Mix_VolumeMusic(vol * MIX_MAX_VOLUME / 100);
-	return OK;
 }
 
 int musbgm_getpos(int no) {
@@ -147,16 +143,15 @@ int musbgm_getlen(int no) {
 	return bgi->len / 441;
 }
 
-int musbgm_isplaying(int no) {
+bool musbgm_isplaying(int no) {
 	return no == current_no && Mix_PlayingMusic();
 }
 
-int musbgm_stopall(int time) {
-	return musbgm_stop(current_no, time);
+void musbgm_stopall(int time) {
+	musbgm_stop(current_no, time);
 }
 
-int musbgm_wait(int no, int timeout) {
+void musbgm_wait(int no, int timeout) {
 	while (timeout-- > 0 && musbgm_isplaying(no))
 		SDL_Delay(10);
-	return OK;
 }
diff --git a/src/cdrom.bgm.c b/src/cdrom.bgm.c
index feeca4a..10cf1bd 100644
--- a/src/cdrom.bgm.c
+++ b/src/cdrom.bgm.c
@@ -42,7 +42,7 @@ static void cdrom_bgm_reset(void) {
 }
 
 static bool cdrom_bgm_start(int trk, int loop) {
-	if (musbgm_play(trk, 0, 100, loop) != OK)
+	if (!musbgm_play(trk, 0, 100, loop))
 		return false;
 	current_track = trk;
 	return true;
diff --git a/src/music_cdrom.c b/src/music_cdrom.c
index 2a2151d..f1d163e 100644
--- a/src/music_cdrom.c
+++ b/src/music_cdrom.c
@@ -58,7 +58,7 @@ int muscd_init(void) {
 int muscd_init_bgm(DRIFILETYPE type, int base_no) {
 	muscd_exit();
 	prv.cddev = &cdrom_bgm;
-	return musbgm_init(type, base_no);
+	return musbgm_init(type, base_no) ? OK : NG;
 }
 
 int muscd_exit(void) {