This is what I use to play local media files.
[{"id":"4fb7f41ac04ccb3c","type":"api-call-service","z":"d0505e55.8a4b8","name":"Play","server":"","version":5,"debugenabled":false,"domain":"media_player","service":"play_media","areaId":[],"deviceId":[],"entityId":["media_player.living_room_speaker"],"data":"{\"media_content_id\":\"media-source://media_source/local/test.mp3\",\"media_content_type\":\"music\"}","dataType":"json","mergeContext":"","mustacheAltTags":false,"outputProperties":[],"queue":"none","x":1080,"y":320,"wires":[[]]}]
The test.mp3 file is saved in /usr/share/hassio/media/.
