diff --git a/doc/usage/cmd/sound.rst b/doc/usage/cmd/sound.rst
index 2cfe9b7..97d610f 100644
--- a/doc/usage/cmd/sound.rst
+++ b/doc/usage/cmd/sound.rst
@@ -1,6 +1,9 @@
 .. SPDX-License-Identifier: GPL-2.0+
 .. Copyright 2022, Heinrich Schuchardt <xypron.glpk@gmx.de>
 
+.. index::
+   single: sound (command)
+
 sound command
 =============