I'm not aware of a standard. This is mostly due to the fact that keyboard shortcuts are very under utilized in web apps. So the specific keyboard combo probably isn't too important. However, your selected shortcut alt + s is very close to the "save" (ctrl/cmd + S) shortcut in most systems and that could be a usability issue.
In my opinion, the bigger issue is removing the roadblocks to using a shortcut key. For example both the current browser window AND the input field/textarea must have focus. Simply being visible isn't good enough.
An additional consideration is what the browser/OS will require to enable a microphone for dictation. Does your user need to provide hardware permission after clicking the shortcut key? If so that will require mouse interaction and you've lost all value from a shortcut.
So like Michael said, you need to provide training near the icon and highlight that when its available.