Sorry, it was my sound level in-game (about 2.0 IIRC at which stage the turbo sound seems to disappear) - I was running it lower than normal because my media player's (foobar) volume can't go all that high. Therefore I run a high level on the windows setting and I don't like adjusting too many settings every time I want to do a few laps/listen to music. I should have checked. I got around this with the seperate waveOut and master volume settings now though.
On a simmilar note, is it possible to include a setting for the system sounds volume?
EDIT: Clarifying - LFS system sounds - ie. the message sounds and the menu clicks and so on.