Pent 4 Windows XP SP2
2.66 GHZ
1.5 GB Ram
Nvidia GE Force 6600
Legacy Audio Drivers
Creative SB Live

Flying the scaled down P-51 65" with EVO 40 engine chosen and at RPM engine range starting at 2000, only other adjustment was a dry weight of 7.75 lbs.

Just tried the trainer, same issues and rescaled the P-51 to sets sounds again under the edit aircraft settings, same results. Another observation which may help. The sound will vary with the throttle, however when you bring the throttle to idle, the sounds mutes itself to a lower than normal level. You can still hear the sounds, but it's muted a few levels lower than just before when you were in flight.
Additionally the sound still will not vary on speed passes or with distance changes.

That's all I can find, and like I stated it happened right after I loaded 1.5. The sound varied with distance like it should with 1.4. Hope you can resolve and thanks for the quick response on RCU. I just performed a system retore for a few days back to remove the 1.0.5 patch and the sound is back to normal.