And you get all other sounds? Just not primaries?
Things to try. In Knossos click the dropdown under MediaVPs and click Verifiy File Integrity to make sure all your files are correctly downloaded and installed.
Check your audio setup? Maybe it's possible the primaries are being lost in a center channel or something?
Ok, so, this is a bit of a laugh. I updated, checked everything, triple checked sound card software, settings, reset config, you name it. Worked it out in the end... For some reason my in-game options for sound effects have been changed (I haven't touched those since... a long, long time ago). I always had all effects / music / voice settings at about half way, as I control my overall sound volume through my headphone amp and half way is a good medium for me. For some reason, my effects were set at 2 dots, and music was at 3. Also for some reason, 2 dots equals 'mute' for some sound effects...? I set it to 3 dots and instantly I was deafened by my primaries and lock on sounds again! Back to 2, silence.
Seems like some oddities with those ingame effects settings, really strange. You can set music down to 1 dot and it still plays fine, but yeah, effects at anything less than 3 causes only some of the sound effects to mute. So all fixed, I have it all set up around half and all is good in the world again