In Qt there are "Media Foundation" and "Direct Show" plugins for the Windows audio. Either of those doesn't have the balance control implemented. It looks that the balance control would be rather straightforward to implement to the Direct Show plugin at least, but as far as I know Direct Show is somewhat deprecated and is in Qt mainly for Windows XP support. See: http://msdn.microsoft.com/en-us/library/windows/desktop/dd389532%28v=vs.85%29.aspx. Qt already implements the wrapper for the IBasicAudio interface and it is used for the volume control. IBasicAudio seems to control the balance also.

For Media Foundation it seems that IMFAudioStreamVolume interface is the one that controls the volume levels of the audio channels. See: http://msdn.microsoft.com/en-us/library/windows/desktop/aa378344%28v=vs.85%29.aspx. That would need to be implemented to the 'wmf' plugin to the MFPlayerSession class.