Sound Notes

vol=75
chan=`amixer controls | grep Volume | head -n1 | cut -f1 -d',' | cut -f2 -d'='`
amixer cset numid=$chan -- ${vol}% > /dev/null

Plugging the 3.5mm jack into the HDMI display.

Note that there might need to be some further setup to get things working consistently. 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):

vcgencmd force_audio hdmi 1

This can be added to the rc.local file so that it's executed at bootup.