You would need to open the back of the TV, disconnect the wires going to the internal speakers. Connect a speaker to line level converter to those wires. That would give you a stereo analog audio output for the speakers.
The only other way is to use an HDMI input selector with audio extraction. Connect all your sources to the switcher with HDMI. The HDMI output of the switch goes to the TV and the audio output of the switch goes to the speakers. This is a fixed level output so you would have to control the volume at the speaker. This won't provide an audio from the internal tuner of the TV.