For the audio preview to work, the URL needs to point to the actual m4a sample file. You get it by opening the developer tools in your browser (most likely via F12), then start the sample playback, find the file in the network tab of the developer tools and right-click on the media file in the list, copy URL.
I feel this works better with a gap of 6720. Playing with [performous], the sync runs-out at the end. Using a beats of 326.68 fixes this. There's, uh, quite a few golden notes too