They will not of restricted audio file transfers, it will be something else. Perhaps your settings for which program is used for audio files has changed.
Try uploading the file/song to RapidShare. The ISP wouldn't be the issue IMO.
I agree the ISP does not sound like it is the problem. Does the file appear to be sent is it hanging in the upload process. How are you sending the file, there are many things that can be the cause.