You have two options
1. Connect the headphone out to the portable input on the Magnavox. You should need a cable with 3.5mm stereo mini plugs at both ends.
2. Use a coax to optical digital audio converter like this one
http://www.amazon.com/C2G-40018-Coaxial-Toslink-Converter/dp/B0002J2MV4
The coax audio out of the TV connect to the converter with a coax digital audio cable. The optical out of the converter connects to the optical input of the sound bar with a toslink optical audio cable.
The digital connection should sound better in theory but in this case I doubt there would be any advantage. The headphone connection would turn off the TV speakers while the digital connection would give you the option of using the TV speakers or...