diff --git a/src/mixer/MixerInternal.hxx b/src/mixer/MixerInternal.hxx
index 585e3cf86..47d0c3507 100644
--- a/src/mixer/MixerInternal.hxx
+++ b/src/mixer/MixerInternal.hxx
@@ -72,18 +72,24 @@ public:
 	/**
 	 * Open mixer device
 	 *
+	 * Caller must lock the mutex.
+	 *
 	 * Throws std::runtime_error on error.
 	 */
 	virtual void Open() = 0;
 
 	/**
 	 * Close mixer device
+	 *
+	 * Caller must lock the mutex.
 	 */
 	virtual void Close() noexcept = 0;
 
 	/**
 	 * Reads the current volume.
 	 *
+	 * Caller must lock the mutex.
+	 *
 	 * Throws std::runtime_error on error.
 	 *
 	 * @return the current volume (0..100 including) or -1 if
@@ -94,6 +100,8 @@ public:
 	/**
 	 * Sets the volume.
 	 *
+	 * Caller must lock the mutex.
+	 *
 	 * Throws std::runtime_error on error.
 	 *
 	 * @param volume the new volume (0..100 including)