diff --git a/src/InstrumentPlayer.js b/src/InstrumentPlayer.js
index d3cf1cd..718fad5 100644
--- a/src/InstrumentPlayer.js
+++ b/src/InstrumentPlayer.js
@@ -34,6 +34,7 @@ function InstrumentPlayer (outputNode) {
  * @param  {number} note - a MIDI note number
  * @param  {number} sec - a duration in seconds
  * @param  {number} instrumentNum - an instrument number (0-indexed)
+ * @param  {number} vol - a volume level (0-100%)
  */
 InstrumentPlayer.prototype.playNoteForSecWithInstAndVol = function (note, sec, instrumentNum, vol) {
     var gain = vol / 100;
diff --git a/src/index.js b/src/index.js
index 2d1be57..047d83b 100644
--- a/src/index.js
+++ b/src/index.js
@@ -91,10 +91,11 @@ AudioEngine.prototype.loadSounds = function (sounds) {
 };
 
 /**
- * Play a note for a duration on an instrument
+ * Play a note for a duration on an instrument with a volume
  * @param  {number} note - a MIDI note number
  * @param  {number} beats - a duration in beats
  * @param  {number} inst - an instrument number (0-indexed)
+ * @param  {number} vol - a volume level (0-100%)
  * @return {Promise} a Promise that resolves after the duration has elapsed
  */
 AudioEngine.prototype.playNoteForBeatsWithInstAndVol = function (note, beats, inst, vol) {