It's strange that no others have complained about this issue. You can't be the only one using analog output on a Pi 2/3
I actually used Kodi on Raspbian and the very same Pi3 before and audio levels were OK. But not on fresh install of LibreELEC.
I'm fresh user and I like LibreELEC and how it smoothly make KODI working.
But I had the similar audio level issue, however, not over Analogue but via HDMI output.
I have Pi3 connected via HDMI cable to my old TV, where I use remote and drive volume over CEC.
But even with TV turned to 100% volume it was barely audible.
LibreELEC version: LibreELEC (official): 9.2.6 (RPi2.arm)
Uname -a: Linux PiKo3 4.19.127 #1 SMP Sun Oct 25 08:49:28 EDT 2020 armv7l GNU/Linux
This forum pointed me in right direction.
Pottered around xml files a bit looking for some volume level controls .. # grep volume `find /storage/ -name *.xml`
I did those changes in /storage/.kodi/userdata/guisettings.xml which normalized volume over HDMI:
(I'm not quite sure at the moment which one was crucial, mentioned are new levels)
<setting id="audiooutput.channels" default="true">2</setting>
<audio>
<mute>false</mute>
<fvolumelevel>1.0</fvolumelevel>
<defaultvideosettings>
<volumeamplification>1.000000</volumeamplification>
Hope that this will help others.