The "built-in" alert sound in the Gmail Manager plugin I have installed on a 32-bit laptop produces a nice, pleasant chime anytime new mail arrives in my mailbox.
However on a 64 bit Firefox, on another machine, the alert sound does not play. Neither does any custom WAV option. I do not get any error messages of any kind, just deafening silence. The only option that works, in the plugin configuration, is the default system alert.
I have no idea if this is something that's broken in the 64 bit version of this plugin -- seems unlikely, this plugin appears to be platform independent; or something is borked in my 64 bit Firefox install. But I have no idea what it could possibly be. Everything else works, and has no issues making noise.