Use the composite/RCA out of converter box: video to TV and audio to external amplifier/speaker set. If no composite input on the TV use the RF/coax for the TV and 'dial' the TV's audio off.
The TV's DVD audio is hosed unless the TV has an external audio output that works.
The external amplifier/speaker set can be as simple (cheap) as a set of amplified PC speakers or as complex as a surround sound receiver setup. Of course if you have an ordinary stereo system with an auxiliary audio input you can use that setup.