Same problem here: notifications sounds will not change after selecting a different one from the sound selector within Twitter's Android app configuration wizard for the active user. And the preset sound is really damn loud and annoying. Anyone figured out how to do this or any hope on getting Twitter to fix this bug?
I filed an error report today on Twitter's help account. I would reinstall the app, but I'm not sure whether that would fix the problem or not, and I do not want to risk losing all my settings, such as starred users whose updates I want to receive instantly.
BTW, some of these thweets are being skipped, a problem that Twitter is already aware of. And search is damn slow now, has me even waiting over 30 seconds every time, when a year ago it was lightning-quick. Also not all posts are shown for a user unless I manually force an update from the app by pulling the screen down. And lists have similar issues: not being properly updated and such.
I guess that they are moving to a NoSQL-based probabilistic database system due to the large amount of accumulated tweets over the years, and at the same time being too scrooge on hardware resources. I hope that they will change this terrible policy, specially after introducing ads, which should give them some steady source of income to deal with investments in HR and infrastructure. Otherwise, Twitter will be a thing of the past, while we users all move on to the next Google or whatever...