I've found the icon folder:
/usr/share/icons/oxygen/22x22/status/
There are 4 icons about the volume:
audio-volume-high.png audio-volume-low.png audio-volume-medium.png audio-volume-muted.png
These 4 icons are just in the 22x22 folder, so it's pixelated if with use them for more than 22 pixels.
I've found the icon folder:
/usr/share/ icons/oxygen/ 22x22/status/
There are 4 icons about the volume:
audio-volume- high.png low.png medium. png muted.png
audio-volume-
audio-volume-
audio-volume-
These 4 icons are just in the 22x22 folder, so it's pixelated if with use them for more than 22 pixels.