Sound notification does not work in Firefox clients - it works fine in Chrome / Chromium

"just one more thing :slight_smile: "

client using Firefox 35.x and 36.x on Linux (Ubuntu, Xubunto, XFCE Mint) do not hear the sound notifications -- it works fine with Chrome and Chromium on Linux, and Firefox on Windows computers .. i.e. sound is correctly enabled in the chat. We had several questions from diff users on diff Linux platforms about it.

Any ideas?