I know that 'lv10 Volume Control: Failed to find mixer" error well! I had exactly the same problem last year with a RPi and a JustBoom DAC HAT soundcard, and got so annoyed that I provided an Audio patch that was incorporated late last year to fix it! In other words... I think I can help you troubleshoot.
That 'lv10 Volume Control: Failed to find mixer" error is the cryptic message that Emulationstation gives when it cannot find the Audio Mixer on the selected Audio Card. You can find more information about that in the Sound Troubleshooting instructions I wrote on the Retropie website
I see that you are running Ubuntu 18.04. I'm guessing you're running ALSA sound drivers, so I'd like you to do a couple of test for me to get to the bottom of this issue.
Firstly, can you please run aplay -L to get a list of the Audio Cards that ALSA recognises on your Odroid XU4. You should get something like this (based on my research):
(You may have different results, just take note of the the HDMI audio card name and keep it for later.)
This will likely show that you have an Audio Card named 'ODROIDHDMI', which is not an Audio Card type that is in the built-in list of Audio Cards in Emulationstation.
The second thing we need to do is figure out what the audio mixers are available on the ODROIDHDMI audio card. To do this I will need you to run the amixer scontrols -D ODROIDHDMI command, which will show you the names of the audio mixers on the ODROIDHDMI audio card. Based on my research you should see something similar to this:
Simple mixer control 'Analogue',0
Simple mixer control 'Deemphasis',0
Simple mixer control 'Digital',0
(Again, you may have different results, just take note of them and modify the command below as needed.)
Take a note of the names of the Audio mixers as we'll need them later.
The next step is figuring out which audio mixer actually controls the volume. On my Rpi Justboom DAC HAT, its the 'Digital' mixer. You will need to experiment to see what works for you. To test the audio mixer, you need to first make a noise out the HDMI, and then second use amixer to adjust the various mixers until you figure out which one changes the volume of the HDMI.
To make a sound out the audio device you can use speaker-test -D ODROIDHDMI -t wav -l 20 & to run the speaker test 20 times in a loop (and go into the background)
You quickly then run alsamixer to get a nice GUI for the ALSA mixer, which you can then use to adjust the various audio mixer devices on the audio card
At the end of that testing you should know the Audio Card name, and the Audio Mixer name on that card.
So now we need to tell emulationstation how to use these new cards.
As mentioned earlier 'ODROIDHDMI' is not an Audio Card type that is in the built-in list of Audio Cards in Emulationstation. Luckily this isn't an issue for us, as the patch I added to emulationstation last year allows you to add a Custom Audio Card, and a Custom Audio Device.
To add a Custom Audio Card just follow these instructions and replace the <string name="AudioCard" value="default" /> line in the es_settings.cfg file with <string name="AudioCard" value="ODROIDHDMI" /> . This should allow emulationstation to successfully look for an use the ODROIDHDMI Audio Card.
To add a Custom Audio Mixer just follow these other instructions and modify the <string name="AudioDevice" value="Digital" /> line in the es_settings.cfg file so that Digital is changed to the Audio Mixer name you discovered above with your alsamixer testing.
Please note that if you change the Audio Card or Audio Device settings from within the Emulationstation GUI at any stage, you will lose your custom settings, and you will need to readd them in es_settings.cfg again as described above.
With the combination of settings above you should be getting sound when you go into emulationstation.
Now I've got something to ask of you. As you have an ODROID XU4 and I don't, can you please reply to this message with the output of the aplay -l and amixer scontrol -D ODROIDHDMI, so that I can see what the audio dcard and audio mixer names are, as I would like to submit a new patch for the ODROID, so that ODROID users in the future will be able to just select the ODROIDHDMI option from the list.
That makes total sense. I have a PXE boot setup at home through which I can (most of the time ;-) automatically image my dedicated RetroPie NUC, etc. so it fits my setup very well. Main benefit I see is that it provides a sort of stable starting point for these other scripts that tune the Ubuntu based RetroPie setup.
I do exactly the same, using a combo of pfsense/unRAID for all of the heavy lifting.