There is no sound countdown before the fight starts in raids sent by other players (I'm not sure).
I tested myself /pull 10 and MRT pull button and sound countdown works in both cases. But I got no sound when I asked my friend to send /pull 10 (he uses DBM).

Did you try having DeadlyBossMods as the only enabled addon and everything else (especially something like ElvUI) disabled?
In raid - no, in my tests - yes

Which version of DeadlyBossMods are you using?
11.0.1

Was it working in a previous version? If yes, which was the last good one?
In versions before TWW prepatch, I'm not sure which one

Sounds like a bug with way they are sending the pull timer. they are probably using an obsolete method of sending it that isn't sending a blizzard countdown. the old sync based pull counters are no longer supported by either DBM or bw and they actually just use blizzard countdown now (/countdown) if they are using something that sends old syncs, it will do nothing

so right now only the way to send pull timer with sound countdown is /countdown ?
and there's no way to change that now?
Only to ask everyone (both people and addons) to switch to the /countdown ?

no, /pull also works, from DBM and BW, what i'm saying is whoever is sending pull is probably using a crappy mod like MRT that's not updated to send them properly. send them with DBM or BW, not MRT, or any other addon that's sending them an old incompatible way.

I retested. My personal counts are always with sound, no matter how sent.
Checked with a friend some more. We both had no addons only with DBM enabled (me) and DBM / BW / no addons at all (my friend). Sent via chat via /pull and /countdown. I never once heard the voice countdown when my friend sent with DBM, with BW or without addons.
My friend heard countdowns sent from me with BW enabled.

The group leader was always the one who sent the timer
I can double-check that the sender is an asist if it affects the count.

the person sending them has assist right? if they don't it's not sent to group just to themself

leader automatically has assist. I cannot think of any reason for problems, unless leader is just purposely running an older version of DBM or Bigwigs

No, the latest versions of addons were used, both mine and hers.
There was no sound when the leader was without addons either.

Found cause, i couldn't reproduce until sender was cross realm. that's what broke in prepatch and issuing fix shortly