The audio has to come in through the HDMI connection on the Sonos Arc. There isn’t a way to stream directly to the Arc. However, you should be able to get dobly atmos through your TV. Dolby atmos content is data that comes across through two different codecs. TrueHD is the lossless version, and that requires an eARC connection. Dolby Digital plus is lossy and can be sent over ARC. Several streaming services have DD+ atmos content, including Disney +, Amazon, Netflix, Tidal, etc. You may have to pay for an upgraded service to get atmos content. Most of these services should have atmos available through your TV smart apps, or you could use a streaming box.
The alternative is to get an HDFury Arcana, which is a device that will allow you to connect a source and have the audio sent directly to Sonos Arc, bypassing any TV processing. That is eARC, so you can get TrueHD Atmos audio without having an eARC capable TV.