How install libreElec in this tvbox?

You can try booting the Tanix or Beelink 'box' devices on the Allwinner/H6 download page.

LibreELEC is an appliance-like Linux. There are add-ons for most media-related things that users might want (other than piracy tools) but there is no package manager so you cannot just install things.

You can experiement with Pulse audio to send audio over the network, but this generally sucks. There is no default/native option in the OS or Kodi to route audio to a UPnP target. Kodi can play from UPnP sources, but not 'cast' media to a UPnP target.