858 is the standard Htotal for 480p so it’s hard to understand why that would specifically cause problems. In that case you should be able to replicate the issue using the test pattern (which is 480p). To get audio playing on test pattern, connect some audio source to one of the 3.5mm plugs or SCART (without video), then select respective AV input and toggle TX mode to DVI and back to HDMI.
To answer your earlier question on 1440px width, I meant using the DTV mode with default sampling settings (720 H.active). That should be identified by 1440×960 by the TV like with that CRU mode or in the video on the thread you linked.