vowel / tone contrast practice.
is there a section where I can practice this? Like a practice module where I can choose to work on listening for the difference between ai and ay and ăi, or to practice hearing the various tones?
If it doesn't exist, could a feature like that be added? As a lingot feature maybe? maybe even allow me to select which vowel i want to review and it will search through my completed modules for audio clips that contain those vowels/vowel combinations and make a review test out of it.
The "odd sentences" in the early lessons are meant to fill this role. If you are looking just to drill the sounds, you will need another resource. In the resource post here, look for the link to the DLI Course on Livelingua - the pronunciation module does exactly what you are asking for.
I'll have to check that out. but i still think this would be an easy setup for duolingo to offer. A simple search algorithm through the texts available and the rest is just like the existing 'strengthen skills' quizzes except that it would focus on audio primarily.
I mean, it is not like there is a whole lot to spend lingots on at the moment anyways.