If you want to hear 5.1, you will need to connect your audio devices directly to the soundbar. A receiver is not a good idea as you will the have an extra layer of electronics that will not enhance or do anything constructive for your sound. Also, with a receiver, you would still need to connect all of your devices first to the receiver and from the receiver to the television for picture and receiver to the soundbar. Again, the only way for your to get 5.1 from a receiver to the soundbar would be if the receiver had a digital output (doubtful).
When connecting devices to an HDTV via HDMI, the TV will only output 2 channel sound to an external device. Stems from an agreement about ten years ago with the music industry covering Digital Rights Management. Thank you Sony.