I would try following method (because this worked for me almost every time, when using different PC-Hardware):
If you are using a nvidia graphics card, then activate nvidia-drivers by editing the file recalbox-boot.conf (just open it with Notepad++ for example) located in the root of your usb-drive, where you installed batocera on and remove the # in front of the line #nvidia-driver=true.
Then boot the system and change Audio Output device to AUTO (not necessary, if you booted a fresh new install).
Reboot and then start Kodi.
Within Kodi you go to System Settings, Audio, Audio Output Device and select the appropriate HDMI-Output device (you can tell if it is the right one by the fact that possibly the monitor or TV name is included, otherwise just test all available output devices within kodi by e.g. using FilmOn or Youtube Addon, to see, if you get sound there).
When you have found the right output device, remember the number of the working audio output device (for example "HDA NVidia, SAM SyncMaster on HDMI #1").
Then exit kodi, go to ES Menu, Sound Settings, Output Device and select the same number (in my example HDMI1).
Then go back and you should hear a "jingle-sound" and a message appears, that you need to reboot the system.
But you do not have to reboot. The sound should already work (except the background music of ES Menu, if you set it to enabled, you will hear this music after reboot).
So just start a game and check if you have sound.
If the sound bar is at 0% just ignore it. Sound should work anyway.
There is another method, involving alsamixer. Somewhere in the forum someone posted a step by step guide how to use it. Just use search function in the forum or via google.
Otherwise, if i have some time later, i will try to find it and post it here.