We currently only offer support for .AIFF, .WAV and .MP3 files up to 500 megabytes in size. If your audio is larger than 500 megabytes, or not one of the three audio formats listed above then your audio file will not upload.

It's important to note that simply changing the extension of your audio file to one of these formats will also not work. For example, you cannot rename a track titled "song.MP4," to "song.AIFF," and expect it to work. To allow our engine to master your track it will need to be exported or re bounced out of a digital audio workstation as an .AIFF, .WAV or .MP3 file.

If this isn't the issue then you might want to try using a different browser / device, or clearing your cache from your browser.
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If you do not know how to clear your cache, please refer to steps included in the article below: https://www.lifewire.com/how-to-clear-cache-2617980  
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This will typically always fix the problem.