Spacehey new chat message sound per abzXYZ

I often miss new messages when multitasking while chatting with people on Spacehey, so I made this addon to play a ping sound whenever a new message appears in the chat and the chat tab is not focused. It works only if you have a chat with a specific user open.
The bell and comment icons (images) are from the Silk Icons Pack created by FamFamFam and published on CC BY 4.0 license.
The ping sound is a modified (cut and sped up) Notification Up II sound created by FoolBoyMedia and published on CC BY-NC 4.0 license