You need to be well familiar with cloud computing as well as REST services, API Gateway, DynamoDB.
Set up AWS a cloud account, allow users to sign up with Cognito from mobile or PC devices
Built a database for the text strings that will be send by the mobile App or the PC and organise them in folders and categories as indicated by the sending device.
Add a tags to the string that contain necessary auxiliary information as :
who created it and when / which folder and categorie/s it is a member of / if it is approved by the administrator for public download .....
Call Amazon Polly and convert and store the strings to audio files.
All logged-in users should be able to download converted audio files if tagged as“public” by the creator
This is only the first step in an ongoing project - which could give you lots of follow-up work - Mobile App and PC program is done by other programmers.
TO be considered please don't send the usual copy and paste "I read your requirements and will do the job in 3 days"