Update: This is now fixed in iOS release 1.9.3! Please update if you're seeing this issue!
Other fixes in 1.9.3 can be found here!
We've noticed that some users on iOS are missing the ability to play sound on their speakers while in a voice channel. The button seems to flash for a second and then disappears:
We're aware of the issue and we hope to address this soon. Thanks so much for your patience while we address this!