Online Audio Converter - repost

Looking to set up an online audio converter service. Users will be able to upload audio and convert it from one form to other, similar to this site; [login to view URL] I would like to set it up as a paid service and be able to have full control over setting u the packages. I would like it to be based on a credit system for conversions. For example, users may get 5 credits per month (which gives them 5 conversions) for registering with their email address and then the packages would offer more. The types of audio I would like to convert to include mp3, WAV, WMA to start with.