if its a fresh install of Ubuntu 18.04 and what ever the icon theme you use those come by default here is the path
/usr/share/icons/Adwaita/scalable/devices/
here is the folder in my Ubuntu 18.04
i have re-placed my customized icons in place of those 4 icons. and below is the result when i increase the volume level.
if you use different custom icon themes (those are not installed by fresh install of Ubuntu), and if that theme has specifically have these four icons, audio-volume-high-symbolic.svg and other 3. then they will be used, other wise above path is used.
in your case, you can simply search for the icon that is bothering you in the above path or in your custom icon theme and put your own icon.
i am using 2 customized icon themes like in below.. one is having those 4 icons and other one not.
if its a fresh install of Ubuntu 18.04 and what ever the icon theme you use those come by default here is the path
here is the folder in my Ubuntu 18.04
i have re-placed my customized icons in place of those 4 icons. and below is the result when i increase the volume level.
if you use different custom icon themes (those are not installed by fresh install of Ubuntu), and if that theme has specifically have these four icons,
audio-volume-high-symbolic.svg
and other 3. then they will be used, other wise above path is used.in your case, you can simply search for the icon that is bothering you in the above path or in your custom icon theme and put your own icon.
i am using 2 customized icon themes like in below.. one is having those 4 icons and other one not.