I used to use an AV receiver and then ramp up the volume on the center channel. Didn't need to even be 5.1 audio as the processing that plucks the speech out based on frequencies worked good enough.

Having said that, it turns out that I'm reasonably deaf in one ear. It's considered "Mild" hearing loss but wow is the effect substantial. My wife listens on videos on her phone at the same volume level that is literally me being able to JUST hear it. Moderate-Severe hearing loss must be impossible. And one of my ears is totally normal... only one ear has "mild" hearing loss.

Anyway long story short, consider getting a hearing test :)