In general it is not possible to control the volume for Dolby Digital encoded audio playback via S/PDIF out because the audio is encoded. You can try setting the default playback device of the system to S/PDIF out, then playing a Dolby Digital (AC3) audio file in your application. You will find that you cannot control the volume from the PC.

To adjust the volume for DDL and/or DTS sound, you need to adjust the volume via your application or speaker system.