CSdoesnotwork wrote on Mar 1^st, 2014 at 11:23pm:

I fixed my sound issues with the custom sounds buy purchasing the 707 and using that sound .dll file as described in the forums.  I recently had to uninstall FSX and then reinstall it.  I did the same things again, replaced the .dll and made the other changes noted in the forums as I had the first time I installed the 727.  Now, after a few minutes the sound to the APU just stops, everything appears to be running.  I can restart the APU and get the sound, then after a few minutes, it stops again.  I have searched the forums and can not seem to find an answer on the APU stop.

The 707 doesn't have an APU, so the reason for the APU sound stopping might have something to do with your using the 707 sound.dll.