I used to play the game when it first came out, than took a loooong brake and only tried to return to it now. Unfortunately, after I install it on my Mac, I cant run it. When I launch it, the Endless Legend icon appears on the doc for a second and disappears, followed by a dialog that tells me it crashed. I tried a lot of workaround I found online and found one that works (32bit beta), but that one has issues with sound (I only have sound during initial logos and when I go into sound options, it tells me that the game "failed to initialise the sound system"

Anyone has any clues how to fix it? Or get the sound working in 32bit beta?

Have you taken a look at our troubleshooting thread for Endless Legend? It contains lots of tips that might help:

https://www.games2gether.com/endless-legend/forum/23-mac-os/thread/6660-mac-os-troubleshooting