Apologies if everyone already knows or have tried this but some tips to help hear dialogue on your TV
try turning on dynamic range compression in sound settings
try turning on night mode (does much the same thing as above)
if using a 5.1 or 7.1 or other x.x sound system, boost the centre channel (or even better -reduce all channels except the centre)
try lowering bass and/or raising treble.
Some of these might help or might not - sound on a lot of stuff on TV is really poor.