RFactor: F1 1979 Sound Update - No Engine Sound!

After installing the 1979 F1 Mod sound update file
(F11979Demo-soundupdate25.08.06.rar) in RFactor 1.15, the mod no longer
has any engine sounds. It has the wind sound, the road noise sound and
impact/collision sounds - just no engine sounds.

The other mods/cars seem to have all of their sounds intact and functional.

Anyone have a fix for this? Is there a way to undo just the sound
update install?

My system runs on WinXP SP2 and has a Turtle Beach Santa Cruz soundcard
if that matters.

Thanks!