^ not sure how everything is setup on your end but in my experience, it doesn't matter where i play the file as long as the audio is bitstreamed in it's original format, i get the exact same playback loudness no matter which playback device.
if your PC is decoding the audio stream to PCM, it's possible that it may be applying effects as well (such as dynamic range compression, volume compensation, maybe even digital volume gain...)
Dolby soundtracks usually have a DialNorm (Dialog Normalization) gain value that is automatically applied by the receiver, and effectively raises the Center channel gain for louder dialog playback. If your receiver is getting AC3 as PCM, then the DialNorm "boost" is not applied and hence, low volume for the dialog.