Document the need to set up ALSA mixer on some laptops

Some systems with integrated speakers like Thinkpad series
laptops require setting up the mixer chip using alsamixer
to make the PC speaker square waves audible.

Fixes: #13

INSTALL.md

@@ -80,3 +80,30 @@ System configuration
     You can test whether the permission setup work by unloading and
     then loading the `pcspkr` kernel module, as loading the module
     will cause all the udev rules to run.
+
+  * On some systems with integrated speakers like e.g. laptops from
+    the IBM/Lenovo Thinkpad series, there is no separate physical
+    speaker for the PC speaker.
+
+	Instead, the square wave sound generated by the PC speaker goes to
+    the integrated speakers through a mixer chip, where it is mixed
+    with PCM sound, CD drive audio, line in, microphone in, and other
+    sources.
+
+	On these systems, you might need to configure the mixer to make
+    the PC speaker audible through the laptop speakers, e.g.
+
+	  * Start alsamixer.
+	  * Find the correct card (switch with the `F6` key).
+	  * Maybe enable loopback.
+	  * Unmute the appropriate PC speaker source (`m` key) and turn up
+        its volume.
+	  * Weirdly, a microphone volume below 0% apparently can also
+        silence the PC speaker beeps.
+
+    Some experimentation might be required.
+
+    Of course, these mixer settings must be saved before the next
+    system reboot so that the reboot can restore the mixer
+    settings. Some systems save the mixer settings as part of their
+    shutdown procedure, others may need you to save them explicitly.