handle all 4 SIDs on Mega65, tape turbo SFX should play on both left and right channels

c64/kernal/init/fda3.ioinit.s

@@ -59,10 +59,12 @@ IOINIT:
 	// Now silence the SID chip(s), depending on the configuration - we want them silent ASAP
 	//
 
-	// First the standard chip (skip if it is covered by whole $D4XX range)
-
 	lda #$00
 
+#if !CONFIG_MB_MEGA65
+
+	// First the standard chip (skip if it is covered by whole $D4XX range)
+
 #if !CONFIG_SID_D4XX
 	sta SID_SIGVOL
 #endif // !CONFIG_SID_D4XX
@@ -100,6 +102,17 @@ IOINIT:
 
 #endif // CONFIG_SID_D4XX || CONFIG_SID_D5XX
 
+#else
+
+	// Mega65 specific handling - it contains 4 SIDs
+
+	sta SID_SIGVOL + __SID_R1_OFFSET
+	sta SID_SIGVOL + __SID_R2_OFFSET
+	sta SID_SIGVOL + __SID_L1_OFFSET
+	sta SID_SIGVOL + __SID_L2_OFFSET
+
+#endif // CONFIG_MB_MEGA65
+
 	//
 	// Now continue the CIAs initialization
 	//