hdmi 'sleep' issue causing first 2 seconds to cutout

* Documentation on how to fix

experiment/production/Sound-Notes.md

@@ -16,6 +16,20 @@ Maybe this in `/boot/config.txt`?
 hdmi_drive=2
 ```
 
+First 2 seconds of HDMI audio cutting out
+---
+
+A known issue.
+I think the HDMI audio 'goes to sleep' when not in use.
+
+I believe the following solves it by making hdmi audio always 'on' ([src](https://github.com/alexa-pi/AlexaPi/wiki/Devices#raspberry-pi)):
+
+```
+vcgencmd force_audio hdmi 1
+```
+
+This can be added to the `rc.local` file so that it's executed at bootup.
+
 References
 ---