Imac connect to Sony's Soundbar

What year model is the iMac...?

see > Identify your iMac model - Apple Support

For a 2017 or later iMac, you will need a USB-C to HDMI adapter.

for example: > USB-C Digital AV Multiport Adapter - Apple

OK thanks for the update on using HDMI.

The other option with the Sony's HT-S400 is the Optical Digital (Toshlink) input. For that you will need to test a USB DAC with Optical Digital (Toshlink) output.