Homepod's were never intended to be used as a surround sound system....stereo yes, but not in a typical 5.1 or more surround sound set up.
As for using a DVD player that needs to have a physical cable connected to another device to output it's sound and picture.....be it via HDMI, Optical, RCA, Coax or whatever.......the HomePod again was never meant to and can't be used in that manner....unless there is a wee box/gismo that can receive cabled audio inputs and output it as an airplay signal....but I don't know or care for such a device.