Well, AFAIK the original game had no sound for UFO fire (and it made sense that way, because it didnt interfere with awesome sounds of player's craft firing...) And also, should we really hear the sound of UFO weapon fire, if we're supposedly the interceptor? It's understandable why we hear the sound of UFO being hit or crashing (since that's obviously quite loud) but there could be an argument why can't we hear UFO firing especially at 60 kilometers...
But then again, maybe it also makes sense for us to hear the sound. Gameplay purposes and all...
Then, why is the laser cannon sound used? If anything, it makes sense for UFO to use plasma cannon's sound, not laser, because.,. well, most likely they are firing plasma at us! Plus there is another argument for using plasma sound... it's less loud!
So, maybe UFO firing sound should be changed to plasma?