You need to set the 'gain' in the filter properties.
I use
Media Player Clasic (MPC), can regain any audio on the fly with it, not just the audio being passed thur the filter(s) that allow gain to be set. From the menu Play > Audio > Options.
CCCP happens to come with MPC.
Other programs will let you select the audio filter or stream filter, and adjust it there.
I've used MPC for so long, I've forgotten how to regain in WMP, sry.
Most of the time the reason the audio is so low, from AC?.? to stero is because the filter doesn't remux the streams the best way, and so 'ambiant' noise is overly-baised to the voice tracks. This is difficult to fix properly on the fly, and so useually ends up with very quiet voice - and normal volume sound fx.