Since MrKid didn't mention this method, here's exactly how I did the drag and drop method, for those having difficulties, if you want to make sure the issue is something else.
0. DO NOT run the Mount Google Drive step.
1. Click the folder icon to open the Files column on the left of the webpage.
2. Drag your audio file on an empty spot in that column(make sure there's enough free space written at the bottom to store your file).
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3. Wait for the file to finish uploading. The orange in the circle is the progress and once it's completed, the filename at the bottom will disappear.
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Then you put the audio filename in the input(it's what the page says to do) or copy it like I did in my previous post which is how I did it.
Weird. I do the exact same thing, and there is never a progress bar. In fact, the circle is red rather than white.
It still looks like this 15 minutes later.