The following messages are printed in the console whenever LibreQuake is loaded with the latest versions of Quakespasm/Ironwail (at the time of writing, of course). In addition, the error related to slimbrn2.wav is displayed as a message whenever the player falls into a pit of slime:

Couldn't load sound/player/slimbrn2.wav
Couldn't load sound/boss2/death.wav
Couldn't load sound/boss2/idle.wav
Couldn't load sound/boss2/sight.wav
Couldn't load sound/boss2/pop2,wav

Additionally, a cdrip is missing for track 0:

Couldn't find a cdrip for track 0