Not sure if the Android App supports Web Audio, otherwise try with a browser first.
You should define the default audio sink in Paper UI via Configuration->System->Audio (or define the audio sink in the PlaySound command as stated in the docs.
Can you select this Kodi audio sink as default sink in Paper UI? And if so, can you try with playSound without the audio sink parameter?
What does your Kodi Thing definition looks like?
BTW, the mp3 file should be in the $OPENHAB_CONF/sounds directory, just to be sure…
2019-03-31 22:21:00.039 [WARN ] [core.audio.internal.AudioManagerImpl] - Default AudioSink service 'kodi' not available!
2019-03-31 22:21:00.040 [WARN ] [core.audio.internal.AudioManagerImpl] - Failed playing audio stream 'org.eclipse.smarthome.core.audio.FileAudioStr
eam@5f707cc3' as no audio sink was found.
from openhab on the tablet it is possible to turn on and off the volume - with no error just some eventslog with confirmation