try the soundbar from another 3.5mm input source (your phone, mp3 player, etc). since they are just speakers you should be able to tell if they are working fine. if they are working fine...then you have choices (below). if they are not working fine then perhaps optical only.
they make hdmi audio extractors with arc support. some have rca L/R outputs some have optical, some have coax. pick what you need. some even have both optical and rca which means you can try both (which makes it a nice choice for testing). i've never used this model but
http://www.amazon.com/ViewHD-Extractor-Support-Converter-VHD-H2HARC/dp/B00LKF6CPG is an example of what i mean. keep in mind that on that particular model ARC seems to only be available for optical out (not rca out). have a look around at the other options as well. just search amazon for hdmi audio extractor.