I lose ATC communication sound when loading PMDG aircrafts of NGX or 777 but the text does scrolling completely, and I also experience same problem for Aerosoft AES, losing sound. All other sounds of aircraft such as engine, warning etc are normally exist. I dont get this problem with other aircraft addons or default FSX aircrafts, only with PMDG addons. is there any different sounds setting in PMDG that makes this happen ? 

Have you tried adjusting the sounds within the sound options menu on the aircrafts FMC? ( I believe you will find it in the PMDG setup option on the FMC)

It maybe a case of having the wrong speaker settings selected. Also having alook at your settings within FSX sound options.

Yes I already set the sound option to 50% all on FMC and FSX. If it's caused by speaker setting or other setting, why do other aircrafts (other addons and default) not create same problem as PMDG did ? thanks