If like me your having problems keeping the PFE/FDC sound volumes stable with FSX Windows 7, try this. It worked for me. Open Control Panel/Sound/Communications, Choose the "Do Nothing" option then Apply. I guess it's reading the ATC as a phone transmission.
Well, good thing I found this post. I didn't have probs with FSX but with FS9s sound (the program itself, not PFE). The volume was cut down after it was loaded. Doing as Toriu suggested fixed it! Windows 7 Ultimate 64bit, btw.