I play almost all the battles tactically given i like melee and the autoresolve seems to have subpar tactics in this area.  I've been losing both sound and particle effects intermittently for a while but i think i've got it figured out now (not had and bugs with it for over 300 turns).

In short the problem is when i try to move faster than the combat system allows.  If you try to give an order or select a unit when it is not currently possible to do so (when a unit is moving etc) it can occasionally glitch out and start losing things.  The more habitually you do this, well, eventually you'll hit a point when the game is vulnerable and it'll just lose the sound fx (+ music in short order).

More specifically, if you trigger a sound effect while there is already a sound effect playing (for example clicking on a unit that plays some soundbite when selected, while an enemy death sound is playing) the game will lose sound more often than not.

With regards to particle effects, for the smoothest play, everything that is going to be lit up by an effect should be on screen when the order is given.  So viewing the whole battlefield when the enemy is taking their turn is a good idea.  If you fire a projectile such as a fireball or arrows and the caster is off-screen, Either no animation plays and damage etc is done as normal, or the projectile animates properly, but flies on from the lower right corner of the battlefield (when orientated to view player force from behind).  Particle effect are usually lost immediately if the first of these possible results occurs.

Unfortunately it appears particle effects going off behind trees and other obstacles dont play at all, even when part of the unit is visible, but i've only seen this a couple of times, so i cant say how this affects anything yet.

While its all very dubious to me about what exactly causes the sound and particle loss, making sure that all particle effects are on screen and fully visible when they begin, taking care not to overlap any sound effects (i believe it's impossible for the AI to overlap sounds btw), and giving the game time to stay ahead of your clicking should eliminate any problems.

Could someone verify any of this please?  I know its very machine specific sometimes so it'd be nice to get a range of feedback.