I couldn't get kodi to output sound over the Tinker Board's 3.5mm jack using any of the easy options, so I had to do it the hard way:
1. Create .config/asound.conf
Code
pcm.!default {
type hw
card 1
device 2
}
ctl.!default {
type hw
card 1
device 2
}
pcm.tinker {
type hw
card 1
device 2
}
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2. Create .kodi/userdata/advancedsettings.xml
Code
<advancedsettings>
<audiooutput>
<audiodevice>ALSA:tinker</audiodevice>
<passthroughdevice>ALSA:tinker</passthroughdevice>
</audiooutput>
</advancedsettings>
3. reboot
Note: If you go into the system settings it will break audio output until the next reboot as there's some bug where simply opening the system settings will force the audio output back to HDMI.