I got a reply to my question on fb, from a retired Viggen pilot. He could not remember hearing the lock sound, and he thought he should have remembered it if he had heard it, since it must have been a recurring sound.
My friend the ex JA 37 pilot says more or less the same, and he added: If you ask Viggen pilots, I think most of them will say "what sound?" :)
The sound is there! There's no question about that. But based on these two answers I'd say that it shouldn't be too audible, especially not with "hear as in helmet" option activated.