When my phone rings the PC-volume is automaticly set to 30% and because of that I can barly her the "Sound on Incoming call".
I can't remember those two functions interfered with each other before (when I used XP).
I'm using 64 bits Vista and MPE 1.7.2.
(Vista has the ability to control the sound volume for all the specific applications that are running... don't know if that maybe will help you fix this.)
It was also in all previous version of MyPhoneExplorer the same behavior - and also on Windows XP. MPE decreases the Mastervolume, if you don't want that MyPhoneExplorer does this then you can disable this feature.