You’re confusing terms here, there is the Arc, which is a speaker from Sonos, and ARC, which stands for Audio Return Channel. The Arc requires an ARC signal to operate. Currently, you have no device in your system that is creating an ARC signal, which is normally created not by the source, but by the TV.  While there doesn’t appear to be a device right now, you may want to read the discussion here which will, when it is released, allow you to do this configuration that you want. I’d assume that once HDFury releases their version, there will be other copycats along to choose from relatively quickly, but the HDFury device appears to be the only such device being discussed publicly.

The Sonos Arc can only receive audio through HDMI ARC/eARC or an optical cable.  ARC stands for Audio Return Channel, which essentially means audio is returned from a display device to the audio device.  It is not the same as audio over a normal HDMI connection.  In other words, you can’t use a  HDMI splitter to send HDMI video to the PJ and HDMI audio to the Sonos Arc, as the Arc can’t read that audio.

The cheapest option is to get an HDMI extractor, that will take the audio from HDMI and extract it to an optical signal.  However, optical can only handle an DD 5.1 audio signal, not DD+, TrueHD, or atmos signal.  You won’t get the full potential from the Arc.

The other option is to use HD Fury Arcana. The link has a lot of good details, particularly the ‘Getting Started’ tab, so I won’t go into much detail on that.

Perhaps the best option for you is to get an HDMI switch with optical audio extraction funciton for now, until Arcana is available if you want to go with that option.  Using an HDMI switch will replace the need to use your old receiver to switch HDMI inputs, in a much smaller box.