Hi =PRoF=, thank you for doing a test which helps enormously.
I can now confirm that both the file size and sound length determine whether a sound is audible. We are talking .ogg files here.
I have managed to get sounds around 8500 bytes working but with sound length => 170ms, but anything smaller, length or size, doesn't work for me.
By padding the end of your file with "silence", which is what I did, both increases the file size slightly but more importantly must have increased the file length to more than 170ms.
Foe ease of measurement i'm guessing that it may be safe to say .ogg sound files should be 12KB or greater to be reasonably certain of them working.
If anyone finds this doesn't work then please post so the figure can be revised, after which Paul may be able to include something in the "Help" guide.
Cheers