Thanks for posting... things got really weird. First, none of your command lines worked. THEN I remembered opt-cmd-p-r (P & NVRAM reeset). Bingo, that seemed to fix the startup chime back to what it had been. Been a long time since I used it BUT I was surprised how totally stuck in the mud it was when booted (swear it took half an hour to reboot <lol>). I have never seen the system look so problematic the last time I did a reset. Something else seemed way wrong (my browser was blinking like crazy)... then I remembered I had to also set my nVidia web drivers (I remembered needing to reset the startup drive). NOW it seems back to working normally EXCEPT the damn startup chime was back to barely audible. So I am back where I started.
Takes me a while to remember **** these days, I THINK I had this issue before, did some posts, tried some things, nothing worked. Then one fine day it seemed to correct itself. No idea how, but MAYBE it might relate to booting into winblowz... I'll try that tomorrow.
I think that's the expected behaviour indeed.
Back into High Sierra, not sure why Apple make the boot chime's volume level controlled by the system volume level. It wasn't like the before, and Apple also "fixed" that later.
Anyway, those terminal command won't quite help.
1) Your boot chime is already on, but just the volume is low
2) the command for disabling it won't help
3) the reset command has no effect before boot chime. And even it reset the volume during boot, but once you boot to desktop, then OS set that lower volume again. Which render that reset command useless.
Performing NVRAM reset can help for that particular boot, but in your case, it will disable the web driver. Therefore, far from ideal. Also, again, once you boot to desktop, macOS will set the lower volume again for you automatically.
I am not sure if BootROM 144.0.0.0.0 will help anyway, but you can check your current BootROM version first. If not 144 yet, you may update it and see if it helps.
Other than that, TBH, there is nothing you can do practically. Of course, you can disconnect all other audio output, but only use the cMP internal speak, and set the volume to 100%. However, that's not what a normal user will do.
Or, you can always set system volume output to 100% just before every single shutdown / reboot. But it seems a bit too much, if just want to get a louder boot chime.