termux-notification always plays a sound

Problem description
termux-notification always plays a sound.
Its option --sound has no effect. And there is no option --silent.

Steps to reproduce
termux-notification -c hello
the same as:
echo hello|/data/data/com.termux/files/usr/libexec/termux-api Notification

Actual behavior
Default notification sound.

Expected behavior
No sound.

Additional information

Packages CPU architecture:
aarch64
Subscribed repositories:
# sources.list
deb https://termux.org/packages/ stable main
# science-repo (sources.list.d/science.list)
deb https://dl.bintray.com/grimler/science-packages-24 science stable
# game-repo (sources.list.d/game.list)
deb https://dl.bintray.com/grimler/game-packages-24 games stable
Updatable packages:
termux-auth/stable 1.3 aarch64 [upgradable from: 1.1-1]
Android version:
9
Kernel build information:
Linux localhost 4.4.146 #2 SMP PREEMPT Fri Dec 6 18:02:59 CST 2019 aarch64 Android
Device manufacturer:
Planet
Device model:
Cosmo_Communicator

Not rooted. adb output upon request.

I do not have Android development environment setup to try some simple fix, sorry.

Start by locating the implementation of the termux-notification command and its handling of --sound, then reproduce the issue with termux-notification -c hello. Done means the default notification is silent while the documented sound option behaves as intended, with coverage for the reported command behavior.