To change alarm sound, it worked fine to move a new alarm sound I wanted to /usr/share/sounds/freedesktop/stereo, and rename it alarm-clock-elapsed.ogg.
For matrix clients I have tested Cinny, nheko, and chatty so far. Cinny keeps crashing. Nheko works quite well, but feels quite bloated. I actually have managed to make an encrypted call using nheko, but it usually doesn’t work. I believe being able to add matrix account into chatty was added quite recently. This has not been a smooth experience, and it seems I have to keep verifying the client. Also calls do not work for chatty. I didn’t know about fluffychat, but I will check it out.
How did you install Axolotl? I just tested with flatpak, and it will not open.