(On WIn7x64) In Mumbles settings, messages tab one can change the sounds for messages.
Now, enabling a sound without a sound file associated to it will open a file browse dialog. One can not use the default resources like :/UserLeftChannel.ogg.
Example use case:
I want to use :/UserLeftChannel.ogg for User Left Server as well. However, I can not specify it but only select sound files on my local HDD.
For me on todays snapshot (1 after 1.2.3rc1)