Right now microphone.png (which is a fall-back for icons like
as a symbolic link to audio-input-microphone.png for small
icon sizes (16x16, 22x22, 32x32) only.
There is no reason to not have the symlink for all sizes.
As a side effect, the icon is used as OSD for the microphone
keyboard button and having bigger sizes makes it non-blurry.