Comment 12 for bug 315732

First thing you should try is updating to the latest official release instead of using the old version shipped in Ubuntu. Remove the lightning-extension package, install the required libstdc++5 package and install Lightning <https://addons.mozilla.org/thunderbird/addon/2313>.

The issue with alarm sounds being played one after another is a know issue in the Mozilla Toolkit that is used in Thunderbird, see Bug 110385.