That plugin has nothing to do whether you hear something or not. It adjust the sound level of each channel, which you define.
When your PC is connected via SPDIF, then you need Dolby Digital Live or DTS Interactive otherwise only analog is working correctly.
I notice the Wiki refers to the "Consider Streams" option, which makes "MyVolume save the volume for internet streams on a stream basis". Can you explain what "on a stream basis" means please. There's a problem with BBC iPlayer volume being much louder than all the other sites in OnlineVideos, so I'm wondering if MyVolume can be used to fix that? If so, do I just need to enable Learning Mode, play an iPlayer stream and lower the volume, then disable learning mode? I'm using analog output.