Why is it the ATC recording of Ethiopian Airlines Flight 302 is not yet available online?
From LiveATC FAQ:


  LiveATC provides many of the major airport feeds on the network but most of the feeds are provided by volunteers. For this reason we sometimes have only limited control over where (and when) new feeds come online.


The coverage is not global, and in some countries (e.g., UK) it is illegal to broadcast it (that's why you can't find any UK airport on that site).

Officially, CVR/ATC recordings (transcripts are a different matter) of accidents are not made public, unless there is a civil lawsuit (jurisdiction dependent). See: Does the NTSB provide CVR audio recordings alongside transcripts in accident reports?