alsamixer comes out-of-the-box and has a text-mode "GUI" which works over ssh.
You can use up/down arrows to control volume, as well as numbers from 0 to 10, which means you can turn your mobile into a fancy "remote volume control", where tapping 1 would be the lowest volume, and 9 the highest.
Number keypad mode is available (on android) in gboard, swype, swiftkey, etc. Also many keyboards/roms have the option to show arrow keys.
Use any ssh client of your choice, eg, termux, juicessh (android), or termius (ios/android).
Moreover, if you use termux with the termux-widget (€2.09 on google play, or free if you install via f-droid, since the app is open-source), then you can launch
alsamixer by tapping in the desktop widget.
To make the shortcut appear in the widget, create an executable bash script under
~/.shortcuts/pi-volume.sh with the following content:
# use your pi's IP or the alias configured in ~/.ssh/config
ssh -t firstname.lastname@example.org alsamixer
-t switch is important here as it allocates a TTY.
Now you don't need to type anything in the terminal on your mobile, just tap the script name in the widget.
(I have no affiliation with termux; I am just a user of the app.)