Android app with audio playing capabilities (streaming).

We are a company of psychologist and physiotherapist who need the posibility in the long term for cutomers to login and have access to different areas such as: Practical - Information registered on the specific customer. - History of appointments. - Future appointments or "calendar events" such as courses the customer is enrolled to. Functional - Login with google/facebook. - Book a new appointment. - Kind of wiki/FAQ on e.g. handling bad days of a depression. - Playing audio files with mindfullness exercises. Short term we only need to have a user login and play audio files. If this is succesful and works as expected we will look into adding the modules for wiki and appointments/calendar. But the design must therefore be able to accomodate further additions. Also the structure should accomodate several sections and not be centered around just playing the audio files. Note! This project therefore only includes the build of a version with login an audio playback capability. The are a quite a lot of audio files so we believe that the audio playback must be some kind of streaming. Please advice on what is the best option for this. We have a graphical designer that has designed our entire website, brochures, posters etc. so he will be able to create graphics for the app. I imagine when the app is mostly done in structure our graphical designer can create the necessary graphics. We have some experience with C# projects so it would be preferred if the project is done in visual studio xamarin but not a requirement. This would make it easier for us to add or remove audio files.