I appreciate your offer of help Austin and if the problem recurs I may well take you up on it.

Unfortunately, the computer was custom built, and the original motherboard has been replaced and I cannot contact the original builder.

I don't even know if it has a separate sound card, or whether the sound card is built into the motherboard (I know the previous one was).

I am adopting the "crossed fingers" approach at present , it's worked for me all day today and hopefully will continue to do so.