Updated mention sound - now works cross-browser

This PR removes the MP3 files and references as there is no browser support for MP3 files in Firefox or Opera.

I've replaced to 'gong' sound with a three-tone alert sound for mentions.  We might want to consider implementing multiple formats and branching based on Modernizr.audio values, eg. from Chrome:

```
> Modernizr.audio
Boolean {ogg: "probably", mp3: "maybe", wav: "probably", m4a: "maybe"}
```