For those looking for useful instructions - allow me to demonstrate how to actually answer a question.
The enabling of the audio output on the 3.5mm audio jack can be done by doing the following:
Open an ssh session into the pi, signing in as root and the root password set when setting up the pi (I use putty in windows to do this)
enter:
mount -o remount,rw /flash
This sets it so you can edit the config file.
to edit the config file, use the command:
nano /flash/config.txt
edit the file by adding in the line
dtparam=audio=on
Exit out of nano by pressing ctrl-x, when prompted to save type Y
To set things back into read only mode, use the command
mount -o remount,ro /flash
Restart the pi - this can be done from the ssh session with the command of:
reboot
Once Kodi restarts, you need to change the audio output to the now available option of the 3.5 output.
(That this only enables the option to easily set it as an audio output, rather than actually enable it, makes it that much stupid that the option isn't there by default. To intentionally add a barrier to an option that end users may want to easily select is the exact opposite of what programmers should be doing!)
go into settings - system - audio
highlight Audio output device
highlight/select ALSA;bcm2835 Headphones bcm2835 Headphones.
With that output now selected, you should now hear audio coming out of the jack, and can now judge for yourself whether or not this is an adequate option for your purposes, rather than some developer trying to make the decision for you.