How are you using alfresco? Whether it is the 1. Alfresco explorer 2. FTP 3. CIFS
If it is through explorer then you can customize the alfresco upload component to allow only desired mimetypes. IF it is through the FTP, there also you can check the mimetype before moving the file to alfresco ftp.
Actually most of my users are accessing alfresco through CIFS.
Do you think is possible to configure Alfresco Explorer when user moving files such as .mp3, 3gp etc to alfresco shared folder that files will be blocked ?