Build webservice for existing .Net code

I have built a site that runs on a shared hosting platform that combines audiofiles, based on many selections a user has made. This audio-generation is done with a library built in C++. I need a webservice on my local webserver that takes the post with various parameters and adresses this unmanaged code library that handles the generation and produces mp3 or wav. Then the generated file must be either uploaded back to the site, stored on my home-based webserver, where the webservice will run, and/or emailed to the user that made the order. I need your advise for the best way to deal with the many MP3-files, regarding traffic and storage. In a later stage my site needs to be made as an app, so if you are good this is the next step. You must be willing and able to work with my code, that is not 100% with the highest standards, and be prepared to suggest corrections when really needed.