Its sounding more and more like a memory leak issue somewhere.

Last I heard from Steve @ BFG they should deploy a patch soon that will hopefully resolve this issue.

patch has been sent to steam & gog so it should be uploaded in the next couple of days

I downloaded the new patch a few days ago and it seems to have cleared up all my sound problems. The game even appears to run a lot smoother so thanks for that - at last I can get going on this.

I'd decided to wait until the sound problems were sorted out - having waited around 18 months already for the game I thought I might as well wait until I could actually enjoy it. Now I just need to remember where I was up to!