Adding abstract function for supporting external master level control

pixelperfectengine/src/pixelperfectengine/audio/base/modulebase.d

@@ -192,4 +192,13 @@ public abstract class AudioModule {
 	 * Returns: The value of the given preset and parameter
 	 */
 	public abstract string readParam_string(uint presetID, uint paramID) nothrow;
+	/** 
+	 * Sets the master level of the module or the module's channel.
+	 * Params:
+	 *   level = the new audio level, linear, between 0.0 and 1.0.
+	 *   channel = the given channel, or -1 if module master level is needed.
+	 * Returns: The new level, or NaN if either channel number or value is out of bounds
+	 */
+	public abstract float setMasterLevel(float level, int channel = -1) @nogc nothrow;
+
 }