In Thunderbird 38.3.0 packaged by portableapps.com.
In Tools->Options->General: "When new message arrives" -> "Play a sound" (checkbox activated) clicking "browse" does nothing, and cant enter the path manually either.
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broken?: new-message-arrived sound-selecting
October 23, 2015 - 5:11pm
#1
broken?: new-message-arrived sound-selecting
It's working fine here. And the portable version has no default sound selected (just as local) and there have been zero changes to anything relating to path updating as you move around in at least 2 years.
The only thing I can think of is if you somehow already have an invalid path in there and Thunderbird can't handle opening with it set. Did you set one previously on a path that no longer exists?
Sometimes, the impossible can become possible, if you're awesome!