diff --git a/src/devices/apu.c b/src/devices/apu.c
index c3034dc..58b5c44 100644
--- a/src/devices/apu.c
+++ b/src/devices/apu.c
@@ -85,10 +85,11 @@ Uint8
 apu_get_vu(Apu *c)
 {
 	int i;
-	Sint32 sum[2];
+	Sint32 sum[2] = {0, 0};
 	if(!c->advance || !c->period) return 0;
 	for(i = 0; i < 2; ++i) {
-		sum[i] = envelope(c, c->age) * c->volume[i] / 0x800;
+		if(!c->volume[i]) continue;
+		sum[i] = 1 + envelope(c, c->age) * c->volume[i] / 0x800;
 		if(sum[i] > 0xf) sum[i] = 0xf;
 	}
 	return (sum[0] << 4) | sum[1];