The HD resolution on a regular TV can't be good as far as I can see.
Can you boot with any boot disc, like SlaYers or Auto Installer Deluxe?
Also test if you can FTP to the xbox after about 15 secs from power on.
If the FTP works it is your video settings that are wrong and I do not know how to fix it, even though it can't be that hard really. Maybe Enigma Video Switch Disc will do the job for you. Enigma is real easy to use even if you can't see anything. But the no sound part worries me.
But start with the boot disc thing, if you do not have one or do not know how to use it feel free to ask on usage (*whisper* and PM me if you do not have one */whisper* please dont ban me
).
Also I think that the SmartXX has more than one BIOS bank or am I wrong there? Try booting to the chip OS instead of the bios and put a new bios on another bank.
If nothing of the above works I am afraid it could be so called 'console coma', but that usually only happens to the EEPROM and not the modchip.
No matter if that fixes it or not please report back here, and if nothing did work I will take a look at some info that might be helping.
Hope you get your xbox working again!